Fortnite's not an FPS... More seriously, I think this is meant to imply a game like Quake or Counter Strike ("Skill based") rather than Fortnite - i.e. Fortnite has aspects unlike those other two games that cultivate different skills.

Reactions-based or high visual fidelity are the main limiting factors I immediately think of.

There are many games where these aspects aren’t core to the game but there are for sure many where they are.

I was pretty surprised by PUBG’s success in the mobile space - Then again I sunk an immense amount of time into Ring of Elysium when it came out on PC and that was in many ways mobile-esque in gameplay and visuals.

So there’s probably more crossover potential than I acknowledged in the earlier post. I think a lot of my opinion on the limits of cross-platform (multiplayer) gaming was formed back when Quake 3 got cross-play between PC and Dreamcast. It was an absolute bloodbath.