iMovie: safe to delete Render Files?
Yes, and iMovie can even delete (all) of them for you.

Go to iMovie > Preferences and click "Delete" next to Render Files (last item on the bottom for me).



It will warn you about deleting data, and that it may have to be regenerated in the future.



Inspecting my iMovie library with Finder after this operation revealed that it cleared the Render Files directory and freed up ~60GB from my disk.