Things to watch out for: focused crafters - while you can't take rituals and then add refinements for specializations, you can add refinements for focus items... and focus items can improve enchanted items. The results can easily be absurd.
Thaumaturgy in general - I'd strongly suggest setting specific expectations for time & effort required to accomplish various power levels of rituals, so you don't get some munchkin wandering in and assuming that they can spend half a day making discipline maneuvers of "gathering power" (or, for the creative munchkin, invent 15 different aspects that all completely coincidentally use the character's apex skill to create) and then toss off a thirty shift ritual of doom.
Evocation is - at the refresh level you're starting at - actually much less dangerous, mostly due to the cost of throwing around high-power spells. However, if people can advance to, say, mid-teens refresh, evocation can get rather ridiculous as well.
Another thing to watch out for is stunts - there's some disagreement over how limited a stunt needs to be, particularly in regards to making attack & defense bonuses. Skill swap stunts can also be dangerous - a typical munchkin move is to find a way to skill swap all of their attacks and defenses to a single apex skill, then add stunts that boost that skill in a "limited" set of circumstances... that, on closer examination, turns out to not be actually limiting.
A really obvious example of an overly broad stunt might be "+1 weapons skill when in combat", though most munchkins will go with something subtler - like "defend with discipline when I can see the attack coming", which seems fairly reasonable for a wizard until you consider that, one, if you can't see the attack coming you automatically defend at +0 anyway, and two, physical defense is really about two-and-a-half trappings (defense vs. melee attacks, defense versus ranged attacks, and the one-half is defense versus unarmed attacks - which is included in defense versus melee, but can also be granted on its own, e.g. by the fists skill.)
Alas, much of this advice can be boiled down to "don't let players make hyperfocused characters", and "watch for munchkins", rather than any easily-house-ruleable flaws in the game system.
Oh, and look really carefully at any 'refluffed' powers, or powers taken from OW; while these are often just fine in their original context, many of them are very easily abused. See, for example, the most recent thread about
A Few Seconds Ahead - a power which I'd strongly suggest explicitly banning; while it's okay on a low lore character, it's just silly on any of the PCs that would take it. There are better ways to work the mechanics of a precognitive power anyway.
