Pokémon - Do the math.

Eevee can also learn Baton Pass, which means that if you can keep it safe for two turns it will give those absolutely monstrous boosts to literally whomever you want. The issue, however, lies in either going up against a Haze user (Haze negates all stat changes when used), or having the Eevee getting KO'd before it can actually use the Z-Move. If the Z-Move has some kind of priority that would help immensely, and the boost in stats will help even more once used, but before that you'd have a non-eviolite Eevee which can be OHKO'd by almost anything with decent stats.

I mean, it's still Eevee. There's only so much you can do with 45 base Special Attack, no matter how many boosts you have.