Do players in the same clan, but on PS4 and Xbox One, contribute to clan XP in Destiny 2? I would assume this is the case.
Is there any downside of having a cross-platform clan, other than players from different platforms not being able to play with each other?
It looks like, yes, this is the case. From this Q&A thread:
It is cross platform. I am the only ps4 player in my clan, while everyone else is on Xbox, yet I still see the XP gains from them.
Do you get the rewards from them?
Yep, all the engrams and everything.