These little guys are the best crewmates possible in FTL. Not only do they not need oxygen making it safe to repair breaches, but they DRAIN oxygen from the room they're in, meaning fire is virtually powerless against them, and any enemies that decide to fight them will most likely suffocate to death in the process, making their combat viability comparable to a mantis.
By far my favorite crewmates in vanilla.
My only beef is they don't integrate well with other species outside of the ghosts because of the oxygen sucking, so getting a crew going can be rough if you're starting with one of the Lanius ships. I've never managed to finish a run with a Lanius crew. Personal favorite is getting a boarding crew of a few Free Mantis paired with a medic, pretty fond of the all ghost teams too, especially the ones that cause breaches when they get damaged.
I was mostly referring to vanilla. You throw multiverse into the mix and things get more... complicated.
Oh gotcha, I didn't play vanilla for very long.