If by R&B superstar meaning their following is primarily from their R&B related music then I would say yes. A majority of the acts mentioned have ventured off into other genres. Bruno was always pop who did old school "soul" album, Weeknd hasn't left synth pop in years. Ty Dolla, no. Derulo, don't make me laugh. Miguel is a name that could've been it but he just didn't release enough to stay around. Chris Brown is an interesting case because he's really done it all except have that ONE album or song that stood above the rest. I know he records a ton and a few of his afrobeat songs have caught on but he doesn't really have that one that people could point their finger to.