Element based core kamis are nothing compared to 100% eidolon. Sol is the only one, by which you won't make any mistake, as long as you don't have her
She is useful for all elements.
Andromeda is shit. Her revival is almost useless, because the boss deals aoe damage and kill 2-3 kamis at once, even damage cut is better than revival. Take this event as example, I use Cassiopeia instead of Andro only because of chaos magic. If I had sol, I can forget about cassi and use mordred or whatever, which give more damage output and boss control, and it doesn't matter, which element I'm running.