Sure, but if you're trying to get someone new to play the game with you on Steam, you may not be able to without buying a separate copy on the Epic Games Store. (I'm hoping they at least let people transfer their Steam copy to EGS, but it's still an extra hurdle for anyone who doesn't already have/want an EGS account.)
In the case of Rocket League, which is crossplay already (steam, xbox, ps4) that shouldn't be an issue.
Hardly, you still cant group cross platform.
Psyonix recently added a "Rocket ID" feature which allows you to add people as friends and party up cross platform.
And how about the ability to purchase in-game items now?