For iCloud, you actually can use iCloud Drive via a web interface from other OSes - I think the biggest limitation is actually that you can't share arbitrary files, only certain types of files (Pages, Keynote, Numbers, Notes, Photos).
I could easily go all in on iCloud Drive if I could make arbitrary files available to other users.
Apple also has a Windows client for iCloud Drive, and you can use standard Word/Excel/PowerPoint files and still edit them through Pages/Numbers/Keynote on a different Mac.
I could easily go all in on iCloud Drive if I could make arbitrary files available to other users.