The difference between skullgirls and roa is that skullgirls is filled with deep, beautifully animated characters filled with charm and presented in an interesting world, intricate story, amazing music direction. It also has a top notch ballance team (although it took a long time for it to get there, shoutouts to early sg where it was mvc3 with boobs) and amazing systems.
Rivals is fun but it still feels and looks like a smash clone built in flash. I know it's still in beta but it doesn't have any charm, some characters aren't animated that well still, etc. A nice story and music design will probably come once full game is out but it will probably come all at once and not build on itself like sgs did.
I guess it's similar in that they both get ignored, but skullgirls is the without a doubt the best team based fighter, arguably one of the best fighters ever, and rivals is a flash looking smash game still in baby steps of development