"Rainbow Six Siege" will support cross-play; Ubisoft detailed the game's upcoming cross-play support during the Ubisoft Forward 2021 streaming. Curiously, PS4/5 and Xbox players can play with each other, but PC players are excluded from that pool; PC players can be matched with Amazon Luna and Google Stadia players, but the cloud version represents many players. I can't imagine that they represent a lot of players.

Ubi did not mention why, but it almost certainly has to do with the skill gap between PC and console players. Balancing mouse, keyboard, and controller has been a consistent challenge as competitive games have introduced cross-play, but ultra-precise FPSes like "Siege" are especially problematic when you start mixing input methods.

Aiming with a Siege controller is much more difficult than with a mouse. This is not a fair fight, and it is not a fact that will change anytime soon; I don't see how Ubi could have avoided this problem in a way that would have kept Siege fair and competitive, so it makes sense to avoid the problem altogether by separating the communities.

This decision is in contrast to how Blizzard is handling cross-play in Overwatch, where PC and console players can play together in groups, but the matchups will remain locked to the platform.

To see the rest of E3, check out the full E3 2021 schedule.