Once an RPC joins the party, how well or poorly they perform depends on the exact same things as for the characters you create: level, attributes, skills, equipment, etc.
If you add an RPC to your party who's a lot lower in level than the other characters, they'll probably always be weaker than the rest of the party. But other than that, any RPC should be useful if you give them decent equipment and keep them around long enough to build up the skills and attributes that are important to you.
Keep an RPC in your party and treat them like the other characters, and they'll do fine. Swap them in and out and don't bother to develop them or give them good equipment, and they won't have a chance to amount to much.
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My personal favorites were RFS-81 and Vi, both of whom went to the end of the game with my party.
Vi was a little lower level than the rest of my characters when I added her, but once I got her some good armor, weapons, and spells, and built up her skills, she carried her share of the load just fine as both a fighter and a spell caster.
I had Myles in the party for a while, but I didn't need his thief skills. He was useful as a pure fighter, but his comments got on my nerves after a while.
Once he left and I added RFS-81, I was happy he was gone.