Asking the player of a spellcaster to have at least one Aspect that sort of sums up their philosophy or belief about magic would be a really good idea.
That kind of points to the subtle and the explodey parts of that ability being two separate things for people to buy. Part of the problem is that Conviction and Discipline have to be high for every caster (or at least every caster whose concept includes the idea of 'I'd like my abilities to actually work when I use them'), so anyone who wants to throw a Veil at the level of their game's skill cap is also capable of throwing a Weapon: <skill cap> level face melter.
The other complicating factor, if we just pull the Veil part of Spirit out, it's a 2 refresh ability, which makes it as/more expensive than say Channeling (Spirit) that gets the same functionality
and a bunch of other powerful stuff. Admittedly, the 2 point glamor ability doesn't cost any mental stress, but is anyone really throwing the 4 or 5 veils a scene it would take for that to matter?
Another way to avoid breaking the game with Evocation would be sticking to the Inherent Limitations in the book. You can't use Evocation to affect anything that's not within your line of sight.
That's a pretty toothless limitation, since other than the other type of spellcasting, what works outside of line of sight anyway? For that matter, just about everything else is much shorter range than 'anything I can see' (e.g. same zone for most physical interaction skills, 4(?) zones for guns).
Evocations have a very short duration.
Again, not much of a limitation since that's true of pretty much all in-combat actions anyway. Evo is actually better than other things, since you get the option for multi-turn blocks and such.
I think the problem with Channeling and Ritual is that they're way too cheap. If we break down their cost, they are effectively:
1 point spent on Refinement (refinement is categorically better than other one point enhancers, since for 1 point you can get +2 to your attack rolls. And it stacks!
) You might say 'But focus items are limited since they could be taken away'. I would reply with, 'You can still cast spells without your items (you just can't break the game's skill cap), if you take away sword guy's sword or gun guy's gun, they can't use their skill at all.'
1 point spent that pays for
everything else. Among those things are: adding a long-range, area, spray with Weapon:Conviction attack trapping to Discipline. Adding a stealth trapping that is better than Stealth to Discipline. Adding an ability to create armor from nothing to discipline/conviction. Adding an ability to create a block for all allies in your zone to discipline/conviction. Greatly expanding what maneuvers you can create with discipline/conviction. You get the ability to break any tech you want at range. Each use does cost at least 1 point of mental stress (except hexing), so that's a downside,
if and only if, your combat encounters are going to have more rounds than you have mental stress boxes.
Ritual is even funnier since it basically boils down to: "Do everything anyone else can do (out of combat) using Lore. Also do a bunch of other things that nobody else can do at all with Lore." Again, for 1 point to pay for the actual capacity to do things and then 1 point that must be spent on Refinement (2 focus item slots).
The other problem is that there are downsides to being a wizard, which would help balance them out in most systems. However, in Fate, the downsides inherent to you being you that come up give you fate points. If wizards exploded when exposed to the color yellow, that might be a real downside, but most wizard problems are of the form: 'Aww, you're inconvenienced, have a fate point'.