I don't have any stats on hand for Carmilla, but I can list some pros/cons:
Pros:
* all 3 skills are great for support (NP drain, defense debuff, buff Carm's crit gain while debuffing the target).
* even if her NP drain skill misses, she still gains NP percentage (at lvl 4 this skill gives ~20%, which is pretty good) which could make or break a battle.
* QQQAB setup with decent crits for a 4-star (even better with her 3rd skill).
* Buster NP to kick ass.
* Single-target NP.
* NP super effective against women.
* NP buffs attack & restores 2,000 HP.
* With Jeanne, Waver, Tamamo (& Tamamo Lancer for her charm skill?) you can stall your face off.
Cons
* 4-star stats.
* No invulnerability, dodge, or skill to pierce invulnerability like Assassin Scathach, Shiki, or Assassin EMIYA.
* 3rd skill requires fully ascended with level 5 bond.
* Crits nothing to write home about if you have Jack or Shuten Douji.
* S growth curve, meaning her stats grow most in the middle of leveling.
* Buster NP means no Q NP-Q-Q brave chain, which might be what you want or are used to with Jack.
I think overall, Carmilla is still pretty niche. I'd choose her over the event Shiki & Scathach, but I'm biased. If you have Shuten or Jack you should probably just use them. If not, I would use Carmilla over Stheno. If you need someone to hit hard and all you have is a scary vampire 4-star, let her mock your enemies while using her. (I don't have Stheno or Assassin EMIYA so I can't really say much for them besides their stats/skill utility.)