I think part of it is that they're not pretending to have any kind of quality or coherence in terms of plotting. The show is good at funny banter, attractive actors and spooky Western stuff, which is basically all it needs. And it doesn't take itself too seriously. A lot of dude antiheroes are the Most Tormented and always right because of their unique cynicism, but the show lets Waverly tease Wynonna and Dolls exasperatedly eyeroll at her.

(Side note: does anyone ship Waverly/Wynonna? I'm not even into incest and I'm already like, damn, someone's probably written fic for this.)