Pirate units to me are pretty weak in durability, however, the IPV seems better at destroying fighters than other corvettes, and the pirate fighter is kind of like the best non-hero fighter. The pirate frigate, it can hold against an acclamator but not win, for me, pirate units are for early game(except the fighters) but that's in skirmish. In GC, since you only get one pirate frigate, not the Venator-class in the briefing, it's pretty useless there.(Unless the tech max is 1, which probably will never happen unless you're playing me

.)