I like doing Underfell roleplayers (preferably being Sans but anyone else is fine I guess) but not because he is a Sans. Sans is far from being my favorite character.
I just like roleplaying as UF Sans because I get to pretend to be an edgy jerk to anybody I want. Also, UF has the best angst or hurt/comfort between Sans and Papyrus. And that is one of my favorite parts about UF.
I really have had some good hurt/comfort roleplays while being UF Sans. Other than all this, Sans is not really my favorite character. I would choose Papyrus, Flowey, Asriel, pretty much any main character over him.