iMovie: safe to delete Render Files?
Solution 1:
I believe you can delete them, but to make sure, you can drag them to the desktop, open iMovie and see if it works. If it works, you can delete the files.
Solution 2:
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.