RGT was dire, but IndieVelo is much, much better. The racing, which, unlike Zwift, enables a variety of different and interesting formats, leaves me feeling cold, though; I’ve only done two events but from what I’ve experienced and seen in others’ YouTube streams, the bots which make up the vast majority of the other riders will ride incredibly hard, then seemingly blow up right at the end of the race to let you win or nearly win. For all its faults, Zwift racing usually feels as if you’re racing against humans. I’m not a big fan of Zwift’s gamification, but there are loads of races without power ups, for example.
There’s nothing you can do in terms of training workouts in IV that you can’t do in Zwift.