I faced a few maxed out (that is, full star, max level top gear) Nightsister teams under Ventress. Unfortunately Ventress and Daka are the only two worthy toons. The damage output was just way too low, and I don't feel there were many synergies happening there either.
I think Ventress would work really well in a turn manipulation/assist based team. My main problem with her is that she doesn't get her Rampage bonuses on her own kills. And she needs Rampage for her AOE to be effective. And she needs to kill things with her AOE in order to be REALLY effective. Right now she just isn't worth it. :(