They are basically a second player, able to fight for their own if you decide to put them in a DPS stance, I mostly use Healer stance but from time to time i set them in DPS for bit assistance. But you better remember to have couple med kits with you just in case unless you're running with a class which has healing skills.
Admitted, if you start a new character and summon Nico, they'll basically do the job for you, especially if you immediately feed him with lots of gifts to rank their influence level up.
But i for one prefer to fight mobs myself, so it doesn't really matter.
Only problem is with KOTFE Companions, the lack of customization. That is probably the only problem. I know a lot of armors which would suit Lana (for example) better.