Crossplay is limited, you have to be invited by a PC player into their lobby, and onto the PC servers. You can't run into PC players with regular matchmaking on PS4. I've been playing since launch and never been concerned with KB/M users to be honest. When it comes to latency I ping about 60ms to my closest server and Fortnite is a low tickrate game, about 20hz, but i never experience rubber-banding, or at least extremely infrequently. I play on PS4 and on a wireless connection, you may have just been having a bit of an off night, or the servers may have been having a rough go of it, though i was playing earlier without issue. I do hope this issues don't persist for you, do keep in mind the game is in early access and see constant support and updates.