Discoverability for streams is awful on Youtube. Twitch also has a very different culture surrounding channels and chat. This results in Twitch having a much smaller audience, but far higher engagement.

Right now Youtube is having to write huge checks to lure away big streamers from Twitch. They'll probably do better with that than Mixer.

Atm the optimal strategy is to livestream on twitch then pay an editor to create highlight videos on Youtube.

Live rewind existed on Twitch until very recently, though it wasn't called out in the UX: you just have to click on the person's profile and go to the VOD page. However recently Twitch added a gate where they changed the default "automatically make VODs available immediately" from true to false, as a mitigation strategy against their DMCA volume (RIAA scrapes twitch VODs in volume).