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.)