Croe's Cutthroats aren't as good as Nyss Hunters unless you're playing with a Telekinesis caster, against Hordes, or both. Their
RAT is too low to rely on against anything but Heavies without aiming, their
MAT is not much better. Fiona does not have defense debuffs or movement shenanigans so Croe's crew will either be stationary to aim or on the front lines getting killed. Poison and Backstab are very nice qualities, but you will very rarely actually get Backstab bonuses and you will probably kill anything but the highest armor infantry on a charge anyway. Still nice against Hordes warbeasts, though.
They do not need Roth's Mercy since Croe is just a leader and not an officer. If someone tries to kill Croe himself, he reveals himself to actually be one of the other guys in the unit and stays alive to be the last man standing. Again, Roth's Mercy would be fantastic on the Nyss Hunters to keep their officer alive and
CRA on the unit.
Edit edit: I corrected myself on who will work with whom. I forgot Fiona is a Syndicate/Highborn/Charter caster and not just a Charter caster. So... yeah, Nyss are still better.