And definitely one of the most challenging, especially when you get into the harder variations. Lever pull-ups are basically like a bodyweight row, 1 arms are freaking incredibly hard, muscle-ups are probably one of the most efficient upper body exercises out there (basically a pull-up and dip combined into one move).
Me either, even the close grip supinated version I feel primarily in my back. Heck, even 1 arms I feel (well I should say felt, as I haven't done any in a while) in my back.
I know people who feel them in their arms, and who have gotten decent arm size out of them, but those people also often times have a really hard time building their back.
Personally I'd suggest using them as primarily a back builder, and if you wish you could use them as a secondary biceps builder. Curls are still IMO the best biceps building exercise out there, so if you want big biceps, curl.