Spent a good minute figuring it out, but ES modules are pretty great. Using Node.JS 13 (and the --no-warnings flag) I can use the exact same file on both client and server. Wonderful!

They're wonderful when you get it to work!

Have you tried introducing them in existing projects, for incremental adoption?
I found that particularly bothersome, naming files with a different extension (mjs), or adding the 'use strict' in every module file with Node 12.