Simple question here: does a dispelling effect, like by use of Unraveling the Fabric of X, need to penetrate the targeted spell's "magic resistance" in order to dispel it?
If you use Spontaneous Magic to cast a Touch range level 5 spell, that would mean that low-level spell can dispel anything of level d10+20 levels. Although I am new to this TTRPG system, it seems a bit off kilter to me that my players' fresh-from-gauntlet magi can easily defeat any level 25-30 spell with Spontaneous Magic, even without any extra investiture in Perdo+Vim.
If the above is legally correct, then a similar vein, wouldn't a non-ritual level 50 dispel on touch be capable of dispelling even level d10+65 ritual spells? Why would any magi even bother with Aegis of the Hearth protections if they can be so easily dispelled?
The answer to the first question is a simple no - the spell itself doesn't have any magic resistance (even if cast by someone who does, the spell is the target not the caster).
Your maths is completely right in the second part, and it is relatively easy for hermetic magi to dispel hermetic effects, in theory. But in practice it's not as easy as it sounds - for one thing a touch range dispel is no good in some cases (try touching a magical stormcloud, or a magically created wolf - easier said than done). It's very hard to dispel some momentary effects this way too, as you have to be very fast in some cases, and others have nothing to "touch" (Mentem effects, for example).
Setting aside the touch range thing, if you're using it against another magus remember that they will be casting at you while you try to dispel - and if they are using formulaic magic while you use spontaneous you gain fatigue each time while they don't, and one slip up could spell death. But fast dispels are a useful part of a magus' arsenal and when you're talking about a static effect (like a ring ward) it's definitely easier to dispel the spell than to have cast it in the first place.
In fact there is actually a small school in house Flambeau who have a fighting style based around dispelling magic, they are described in the Hibernian tribunal book. It's very much a valid tactic.
Specifically about the Aegis of the Hearth that's the one unique spell that actually does have a sort of magic resistance of its own. In the description of the Aegis it says "If any spell is cast toward the Aegis (originating from outside it) by any magus who was not involved in the Aegis ritual, the Aegis resists
the spell" that means that if you cast a dispel at an Aegis you have to not only cast a spell high enough level to dispel the Aegis ritual, you also have to penetrate Magic Resistance equal to the level of the Aegis on top of that - not impossible, but definitely not easy. Your example level 50 dispel would need to have 65-75 penetration to work!
I always forget.
If the targetted spell is on a creature with magic resistance, do you need to penetrate the creature's resistance to to unravel the spell?
For instance Mage A has cast "Rise of the Feathery Body" (Touch range ReCo) upon themself and then raised Parma.
Then Mage B casts "Unravelling the Form of Corpus" on Mage A's levitation. Does Mage B need to Penetrate Mage A's Parma (and Form bonus) to drop him out of the sky?
ArM5 p.184 Magic Resistance and Penetration has "Magic Resistance based on might functions like Hermetic Resistance, resisting the powers of all realms, and all types of mystical power."
For Hermetic Resistance see ArM5 p.85f, especially "Magic resistance keeps magic away from the maga, her clothing, and other items that are very close to her."
So a spell effect on or close to a creature is covered by that creature's MR.
OK. For some reason I was under the suspicion that Touch range spells that need to Penetrate the MR (or have Parma lowered) might be handled differently.
There are creatures that can lower their MR (like magi), and those (like ArM5 p.194 Stellatus) that for one reason or another can't.
If a creature thinks that a spell cast on it is going to remove a mystical effect it wants gone, it might lower its MR if it can.
Anyway, the Rise of the Feathery Body cast by Mage A on himself is protected by his Parma, once that is up.
EDIT: Do you mean (ArM5 p.126) Wings of the Soaring Wind, not Rise of the Feathery Body, cast on Mage A? Then things get tricky, because the "massive gust of air around you" can well be considered not close enough to you to be covered by your Parma.