Yes, my main problem it that right now, there is no incentive to every do anything outside of your specialist element. And there is a minor inventive to specialize in spirit, since it explicitly gets the ability to veil, but none of the other elements give you an explicit ability.
Notice how Harry uses fire (and sometimes earth) based attacks, and forces based defenses. This makes him more interesting to read about, but it the game, it makes him weaker. So a Harry in the game wants to use fire for everything, and barring special effects, his magic is almost identical to someone who uses any other element for everything.
If there was no mechanical incentive to specialize, then people could mix up their elements however they wanted, and it wouldn't matter. But there is a mechanical incentive to specialize, so there should also be a mechanical incentive to be a generalist. And, ideally, a way to differentiate specialized wizards who are at the same power level, but use different elements.
So, if we have a sample wizard with discipline 4, conviction 3 and we are going to make him a combat wizard.
He'll take as his evocation specialty spirit power, and get +1 spirit offense and defense from foci.
So he can throw around power 5, control 5 offensive and defensive spells, and can explicitly veil stuff (with power 4, control 4).
We'll take the same stat wizard who wants to attack with fire and defend with water:
Discipline 4, conviction 3; evo specialization in fire power. Focus items for fire offense and water defense.
So he can throw out power 5, control 5 offensive fire spells, but only power 4, control 5 defensive water spells. And if he wants to veil stuff he's only at power 3, control 4.
And when they take refinement, the gap gets worse.
Spirit guy gets +1 spirit power and control with his refinement (getting him +2 spirit power, +1 spirit control, in total); he gets better veils (power 5, control 5), and he goes to power 6, control 6 attacks and defenses.
Fire/water guy gets +1 fire control and +1 water power with his refinement and he's at power 5, control 6 attacks and power 5, control 5 defenses and his veils are still only power 3, control 4
----
And since all elements do everything (more or less), there is no reason to be fire/water guy. Sure, fire/water guy was a very marginal maneuver advantage, since he gets two lists of possibly maneuver aspects to inflict, but with the stress cost of evocations, you aren't going to stack up that many maneuvers on someone. The spirit guy is happy with stuff like knocked down, staggered, and blinded.
Even with my suggested mechanical changes, fire/water guy still isn't that great compared to spirit guy
Since both (with 1 refinement) can throw power 4, area 1, control 6 attacks and maneuvers, but at least fire/water guy has slightly better counterspells (spirit guy would have power 3 control 4 for counterspells, and fire/water guy would have power 4, control 4 (plus maybe foci bonuses)).
And now we compare all fire guy to all water guy. The fire guy can throw power 5, area 1, control 6 attacks while the water guy can only do power 4, area 1, control 6 attacks, but water guy has much better counterspells than fire guy (water guy is at power 5, control 5 countspells (plus maybe foci bonuses); fire guy is still at power 3, control 4). And all air guy can find stuff like nobody's business, and earth guy can make walls of lava that last a bit longer than anyone else, and spirit guy can disappear.
There's still not much inventive to be two element guy, but at least you get a bit of flexibility in exchange. It's probably still not enough though.