The problem with these is that they were created before the whole accelerated abilities / difficult arts introduction.
I may be wrong, but I seem to recall Eric Tyrell saying somewhere here that, if done now, these would be accelerated abilities. This doesn't help, though, as I'm not sure there's a cap (or not) on these.
Do as you like! 
It's an Art - probably shouldn't go above 10 at char gen.
End of story.
Not a hard rule , by RAW , a suggestion for character balance.
Unless the character has an Affinity , then 10 is unlikely.
thus "probably shouldn't"
I believe that you have me confused with someone else
Oups!!!
You're pretty right, wrong Erik there 
I've searched it, though, and found it, which conforts me, I feared it was yet another case of senility from me. Sigh of relief.
So, if I understand correctly, if made now, the Goetic Arts would be marked as Accelerated Abilities, and the Method & Powers as Difficult Arts.
Still, I don't know if there's a max at chargen for Accelerated Abilities. Not, btw, if the age restrictions on abilities are lifted for difficult arts. Maybe there is, but, well, my ars-fu ain't good enough.
Far be it from me to contradict the guy who actually wrote the stuff, but I would note that the Ars Fabulosa were written well after the introduction of Accelerated Abilities and use the same mechanic as the aforementioned Goetic Arts. I suppose this could be for the sake of consistency but I'm beginning to think that perhaps the Magic Realm, due to its disinterest in humanity, allows those associated with it to manifest supernatural powers in a way that is intrinsically different to what the other realms allow.
I dont know anything in this case, but order of publishing doesnt have to be the same as order of writing.
atlas-games.com/arm5/arm5errata.php#AG0285
While this is written for Societates and not Infernal , it should still apply.
My modest proposal - call everything that uses the Art experience table an Art and everything that uses the Ability experience table an Ability. The "Difficult" and "Accelerated" categories are just confusing.
I think this may be for Focus purposes: If foci apply to arts only, it can apply to difficult arts, but not accelerated abilities, which changes a lot of things, while if you don't make that distinction, you're toast.
Hmm.... did not think about this but... Would a Puissant difficult art get a +3 or a +2 bonus? Per the RAW it would be a +3 (which makes Puissant Difficult Art quite a good buy) but I am not sure if that would comply by the intention of the rules...
Xavi
Arts +3.
Is it difficult or not.
So puissant art is always a good buy for difficult art 
Hah! I just noticed they changed the Might of the Forest Spirits to fit the RoPM guidelines. Poor King Fir now has Might 60 instead of 90...
On point: this appears to most plausible interpretation to me, although I'd still use the "recommended" cap of 10 (before affinity), just like an Art.
All the Difficult and Accelerated terminology is a bit confusing. There are several aspects that come into play:
I don't think there is a consistency in the rules in all these points. The key distinction societates makes is between verb-noun as Art and alone as Ability. This is disconnected from the other aspects, which can lead to strange results. I would have preferred Jabir's suggestion, since it meshes better with other rules referring to Arts and Abilities. I would suggest the best way to define things would have been:
This is not, however, how things stand. Which isn't too bad - the accelerated/difficult terminology is clear enough.
The problem with that is that sometimes you roll Characteristic + Ability, sometimes Characteristic + Art, sometimes Characteristic + Ability + Ability, sometimes Characteristic + Ability + Art, sometimes Characteristic + Art + Art.
As it stands, this is simple (except where we screwed up). Characteristic + Ability, Characteristic + Art + Art, cannot use an Art by itself.
There are two fundamental mechanical distinctions, and they now cut across one another. I decided to privilege the way you generate a total, because that was the one I wanted authors to really pay attention to.
OK. Makes sense. By this the Goetic Arts are Arts then.
Did you want Puissant Art to work for difficult Arts and Puissant Ability to work for Accelerated Abilities? Or did you want the +2/+3 to go with the x5/x1 costs? I assume the latter, though the naming convention now seems to imply the former.
Chris
Goetic arts, in David's thread would be abilities. Because you only roll them + characts.
Not. They are rolled Char + Art + Realm or Area Lore (Goetics) or Char + Art + Faeric Rank (Fabulosa), then it's clear that they aren't accelerated Abilities but another type of Arts. In any type there aren't limit about the initial Score on them. Rules say it.
And then realm lore are arts and abilities aswel?
It does not make sense...
I prefer not to try any system. The rules are done without it, and they don't support any.
There are other formulae that use arts and regular abilities combined. Or all of them together. The philosophiae formulas come to mind.
Xavi