Well, you can use Unreal Engine on Mac. As for as I know (I’m new to this), the Datasmith plugin for seemless integration of Sketchup models into Unreal Engine just has not yet been developed for OSX platform.
I’m currently finding my way in working with Unreal Engine on Mac and I believe you can do amazing things as well, just with some workarounds. Though these challenges will only learn me to understand UE better I suppose!