I can see what the game balance problem is, easily.
But many of the restriction people wish to put on them are just silly and arbitrary, which is all my argument was about.
Was this the 4th edition mastery?
By 5th edition RAW, each mastery level reduces the number of botch dice by 1. Casting this spell in a +0 aura and rolling a 0 is 12 botch dice (one for each pawn of vis spent on the spell), and most casters are probably casting in an aura so add a few more. The only way you're going to see this down to 0 botch dice is when it is cast by a Mercurian ritualist who has a decent gold cord strength or cautious sorcerer as well as some time invested in mastery.
In sagas I've seen, the 'what if we botch?' is typically the reason people don't abuse these spells. One or two castings? Maybe. Boosting all stats to 5? That's just asking for trouble. A spell botch is not just some warping points; the spell itself also goes awry. One of these spells going awry should, IMO, be a rather bad experience for the target. Probably to the tune of a permanent characteristic penalty of more than -1, along with a minor flaw. Maybe the caster too.
As you yourself say, God says "no." I see botches being a fantastic place to express that.
That said, I'm sure there are many magi who would try it regardless of the risks. Just probably not all of them.
I've been staying out of the discussion about characteristic increasing rituals because we've been through this argument here before and I don't really have anything to say that I haven't said before. But since we're still talking about it, what the heck...
Direwolf is right that these magics are quite doable and very cost effective according to RAW. Yet, they never seem to appear in the Ars Magic world as seen through published supplements. So we seem to have a conflict between the technical rules and the game world as envisioned. Some will want to chose the rules in this conflict. I would instead choose to retain the spirit of the game world and change the rules.
I have two reasons for this. The first one is general power level. Magi are already overly powerful for my tastes. This is just a matter of taste though; there's nothing inherantly better about either a high power or low power game. My other reason is that I'd prefer not to roleplay superhumans, either PCs or NPCs, in this game. Once you have a Order made up of the supremely intelligent, or communicative, or awesome (i.e., presence), it becomes hard to play them as normal flawed human beings. I want crotchety old wizards who get into misunderstandings and have trouble getting along, not fantastically beautiful, understanding, and social demigods. That's for other games.
You are right, but personally, in practice, I don't see it as being a huge issue.
Say, you want to increase your Com from 0 to +5.
If you are casting the ritual yourself, you need to invent a level 55 ReMe ritual, and then cast it 5 times, which requires 55 pawns of vis. This is all perfectly possible, but it does mean that you are either very highly specialised in ReMe, or an archmagus, or have a team of three or four quite competent assistants working for you in the lab. And it means that you have a lot of vis. You can do it with less vis, if you invent each of the intermediate rituals (one to raise from 0 to +1, one to raise from +1 to +2, etc) but the smallest amount of vis required is 45 pawns, and that requires you to invent five different rituals.
If somebody else casts the rituals for you then it is a level 60 ReMe ritual required, and it requires 60 pawns of vis (or 50 pawns if you step up through the "small" rituals), and you will get 5 Warping Points too. And that's not counting whatever it is that you need to pay the caster.
In a low power saga, this is likely at least two years worth of vis production (maybe considerably more) from the entire covenant, even assuming that the covenant harvests the right sort of vis. It seems unlikley that the magi in a low powered saga will have enough resources to dedicate to this. They might be able to do it eventually by saving vis over a large number of years; but remember this is just for one characteristic, and they might well have better things to do.
In a high power saga, the vis required is less of a problem. It is still probably a reasonably significant investment, and the magi might have better uses for the vis. However, in a high powered saga, it is expected that the magi can do important and powerful things --- that is the point of a high powered saga. So, I can't see why it is an issue there either.
This is not true.
In a peaceful situation, a mastered spell never rolls botch dice on his stress die if a 0 occurs.
At the opposite of formulaic, ritual use always a stress die. That doesn't mean that they are always stressful.
True but a little misleading: in a peaceful situation (when using a simple die roll*), there is never a chance of botch regardless of whether the spell is mastered.
- As pointed out by Cuch and Exar, this parenthetical comment is wrong.
The definition of a stress die includes the possibility of botching, so whether the characters are feeling stress or not is irrelevant: rituals always include a chance of a botch.
Excellently put. I second this fully and whole heartedly.
For those who would think otherwise from my above post, please understand that I was arguing not about the boosters themselves, but purely about the artificial restrictions that some people were suggesting.
Again wrong: mastered spells always use a stress die, even in peaceful situation.
No, Gerg is right (even if you are too) - see page 81, "Formulaic Magic" -
"If the maga is not under any pressure, it is a simple die"
What this means is that all those casual "daily" utility (formulaic!) spells - lighting candles every night, or removing dirt, or scrying on the Duke - no chance of self-destruction, even if unmastered, no matter how many times you cast them.
Once mastered, it's always a stress die, but with rules which mean you cannot botch since there are no botch dice if relaxed-
"...Mastered spells are always cast with a stress die, but if the mage is relaxed there are no botch dice..." (p 86, last par)
What this means is that the Mastered Caster regains a chance at an open-ended roll to attain high Casting Totals, but still not the usual chance of self-destruction - all good.
So, in short:
Unmastered Formulaic:
no stress: Simple Die (+1 to +10)
Stress: Stress Die (+0 (or botch?) to +9 or doubled)
Mastered Formulaic:
no stress: Stress Die, no botch dice (+0 to +9 or doubled)
Stress: Stress Die, -# botch dice = Mastery level (+0 (or (less chance to) botch?) to +9 or doubled)
Funny thing is, you seem to be agreeing with ExarKun 
Cuchulain:
That is wrong. Thus my answer. And as spot Tellus, you are exactly saying what i say 
Relative to how RAW presents "average" power gain and Vis availability i guess is the rough answer.
I´ve tended to be in somewhat Vis poor games(especially after i changed so that any lab work uses Vis or gets penalised(or get a small bonus from using extra, making Vis richness much more desirable and noticeable), because even with low gains Vis was still stored up in big chunks) and the "worst" would be that it took some time to fix.
This is one of the reasons why i changed Permanent to raise the magnitude of spells somewhat(and raised the Vis cost severely to get Permanence), because it seemed "less than optimal" for a permanent spell to be 20 levels lower than one for Year duration...
Exarkun, i would NEVER allow this kind of spell to have anything but Target Individual. I consider that going severe munchkin.
Dont forget that you can develop the low level version much more easily first(because getting the one for +5 right away is going to be somewhat of a strain, both to develop and in Vis for Casting), preferably the highest possible that you can get in 1 or 2 seasons of work(unless a major specialist, usually thats the level 35 one). The similar spellbonus makes a BIG difference when you create higher level ones.
Preferably, you find a NPC or PC that is either a Creo specialist, or one with CrMe specialisation. Unless they´re young they will likely have 20+ in both Arts(or 30+/5+ for a Creo spec.), with some MT( 5-8 ), Int(+3 ), familiar(4-9), decent lab(3-9), ok aura(3-5) and your PC as assistant(6-12), that will often reach a lab total of 70-80 without problems.
Problem then is that they become utterly useless. Raising +4 to +5 would then be a Level 75 ritual. That kind of vis expense and the warping for raising a stat more than by +1, and just the much greater difficulty in developing the spell... No, at that point it has no reason to even exist at all.
Yup, that was a big part of my point as well.
Oh yes. Which is why only relatively few actually DO up stats a lot.
Warp yourself for 5 years to write the best summa ever. The stop being so communicative. How is that useless? 
But well, them being useless is a fairly positive result IMS, so I would buy it. YMDV
Cheers,
Xavi
Um... no.
What exactly do you think is "wrong" with it? Doesn't specify Mastered Spell, so I see only RAW re any standard Formulaic casting.
Nothing "wrong" about that statement. In a peaceful situation with standard formulaic spells, you use a simple die, and so there is zero chance of botch. True = not wrong.
And nothing "wrong" with that statement either. In a peaceful situation, even when mastered there is no chance of botch. True = not wrong.
Now, combining the two together does confuse the dice thing up a bit, but it's still not "wrong", just not as clear as it could be.
(English: The idiot bastard child of incestuous European cousins.) 
Why is this wrong?
- ritual always have a stress die thus may botch, even in peaceful situation (unless mastered, of course)
- peaceful situation is not using a simple die roll, since mastered spell always use a stress die, even in peaceful situation
- non fatiguing spell casting do not use a die roll (neither simple nor stress) in a peaceful situation
- in a peaceful situation, casting a spontaneous spell needs a stress die.
Choose your reason 
.
Most caster would be in a magical aura which has no impact on botch dice ("Aura ... botch ... foreign realm." ArM5 p 183, realm interaction).
And there this crazy last sentence on p 86 "Mastered ... relaxed ... no botch dice". The only way out of madness is to decide that rituals are never relaxed.
BTW, maybe the Ritual Casting Total should be errata'd to be "... + Stress Die" instead of "... + Die Roll"
That has been suggested before - (may even be erratta'd? I know I use it, forget if it's "canon" or no)
No one (except you, just now) is talking about rituals.
He wasn't (necessarily) talking about mastered spells.
No one ( except you , just now) is talking about spont spells (see rituals, above)
(see rituals, above)
I'm done with this.
Really? I could have sworn that rather a lot of us were talking about Instant Creo spells with lasting effect (making them rituals) vs spells that rather quickly pass level 50, making them - again - rituals.
Did I miss something?
Sigh...
No one in the cited exchange* is talking about Rituals...
(* Gerg, EK and myself)
I'm sure someone, somewhere is talking about Rituals.
Better?
out. (no, really.)