When a character is taken out by stress, their opponent decides what the consequences of this will be, moderated by the implied context of the contest.  Seems reasonable enough - a truly unfriendly combat could end with the character dying, while a relatively restrained arm-wrestling conflict might result in bragging rights but no lasting physical consequences.

So how should the mental stress associated with spellcasting be interpreted?  There's no opponent character involved, so no character-desire context.  It's just the GM's opinion.  Which wouldn't be so bad, except... The book describes mental consequences as extremely severe.  I suspect they're talking about the worst possible cases, rather like dying is the worst possible outcome of a physical struggle.

The player can't concede as an action - there's no struggle, and therefore no action other than their own that would push their stress over the limit.  So what happens if someone goes over their own mental limit?

I realize that the RPG was written pre-Turn Coat, and so had a limited amount of canonical material to work with.  The game's authors aren't psychic, able to know what Mr. Butcher intended.  And of course any game system will have limitations.

But as much as I like the game overall, I find myself unhappy with the way that magical practitioners with a specialized focus are address in the mechanics.  Hannah Ascher isn't an ideal example because her talents were being influenced.  However, it's notable that we never see or hear of her casting a spell that isn't directly connected with flame.  'Aristedes' actually has more capabilities than the game rules would permit.  But later-book Mortimer is a fantastic example of how the rules don't adequately represent what the characters can do.

Mortimer is 'only' an ectomancer.  He doesn't seem to have any abilities beyond that - not even Wizard Biology or the Sight, although I would argue that he has the innate ability to sense spirits without consciously enacting magic.  Yet he's described as being, in some ways, more powerful than Dresden - who is one of the top forty wizards on Earth.  Mortimer doesn't use focus items or ritual paraphernalia to any great degree, yet can enact spells that we're told make some of Dresden's look pretty crude.  And he can perform effects that would seem to be associated with Thaumaturgy (which is a slow method explicitly not suitable for combat) on the fly and so rapidly that they do actually impact combats.  One example of this involves temporarily 'imbuing' himself with the skills and powers of specific ghosts - effectively spirit possession in reverse.  And this at a moment's notice, while a madman tries to kill him.  Another is his improvisation of the wraith-firehose, which he constructed as a weapon with only about a minute's preparation.  And again, he managed to draw Butters' spirit back into his empty body quickly enough that CPR was able to keep the body alive.  Without any ingredients, ritual preparation, etc.

The rules for the 'Focused Practitioner' template aren't compatible with that.  It seems designed to represent untrained dabbling in magic, disallowing the options for improvement and specialization that wizards get.  There's really no way to represent a practitioner of magic who is obligatorily focused on a single aspect of the art.