As Erik Tyrell rightfully reminded us, spell guidelines are guidelines not rules. In fact, spell design leaves a number of areas to the appreciation of the storyguide for balance.
In Ars Magica, if, like most people, you do not have Magic Resistance, any successfully-cast spell will affect you. However, a storyguide may decide, on a spell-by-spell basis, to require a "Natural Resistance" check, which may protect even people who do not have Magic Resistance, or allow them to escape its effects.
If you look at existing spells, it can be difficult to decide whether Natural Resistance should be allowed or not. In many cases, it seems it carried over from previous editions of the game.
For example, Clenched Grip of the Crushed Heart causes an instant kill, while Black Whisper allows a (quite difficult) Stamina roll to avoid going insane. Entrapment spells typically allow Strength check to get free, but you cannot "dodge" Pit of the Gaping Earth (which is understandable given the size -- there may be better examples in the books). Is the fact that Mists of Change transform a random fraction of its victims a deliberate design decision or an inherent spell limitation?
What are your own guidelines when deciding when a spell should allow "Natural Resistance" to avoid, cancel, reduce or overcome the spell's effects?
My thought has always been that in fifth ed. the magic happens and a natural resistance roll can't stop it. The natural resistance rolls are about how the target deals with the magic that has happened to them.
Ergo they can pull their brains back together after the black whisper but they can't restart their heart.
Erik - I can see many (munchkinic) players (including some of mine) arguing that their spells should systematically allow for maximum effects implied by the guideline without any chance of mitigating them.
i.e. - no, he's insane, why should he recover before the spell ends?
As a broader defense of yhe position that I presented with regard to this particular criticism my point is only that the spell happens. This in no way implies that there is no variation in the effectiveness and power of the spell. Natural resistance is about the target's response to it. Other factors describe the variation of the spell itself.