Dang and here I thought I could send Erik home, so I didn't have to worry about him. Serana is marked as essential, but if she's that bad I'll have to rethink that one.
I rarely use companions but I didn't find her that bad, to be honnest probably one of the most interesting companions/characters. Granted, my character
is the tank and I don't really sneak, so I don't mind her staying in the back shooting ice pikes, summoning undead, and draining life. If I was trying to be sneaky, it would be a problem I guess, which I think is what the people are referring to.
But I don't always bring her with me and she'll be gone eventually.