There's too many insane 4 drops with an evolve effect which doesn't help when player 2 gets to evolve right on that turn.
I think cards like Siegfried are the type of design needed to curb the evolve advantages. The instant attack hit gets cleaned up without having to evolve in return which is a big deal.
The type of effect that I've seen mentioned is not an evolve effect. It's a reaction to the enemy evolving. There's only 1-2 cards in this game that I've seen with an effect like that I think, and I'm not even sure if I remember that correctly.
Something like "Fanfare: Destroy an evolved enemy" or "Evolve if there is an evolved enemy" could be interesting I think. Although in the first case, you'd be better off with more Siegfried like cards for the consistency.
Problem is that if they are too good, then P2 can use them to get further ahead without spending another evolution. Difficult problem to have.
I could see this being a problem though.