Earlier quoted context omitted.
Dropbox can handle a LOT of files. I haven't seen any other service that doesn't just break. OneDrive cannot handle tons of files. It just doesn't work. It ends up freezing, you have no idea what's going on. Can't speak to macOS related stuff. We finally were able to migrate off of this.. but we had a VERY file hefty folder structure. Instead of databases we chose to keep things in individual files for various reason…
We just had to switch from Box to OneDrive at work, and it's caused massive headaches. Most days no one can log in, and some Excel sheets uploaded have been corrupted or have missing sheets - I don't know how that happens unless Microsoft is scanning our files.
What I've done is tell people to create documents from the "cloud" and edit them there within the Office365 ecosystem. Documentation needs where I work are fairly simple, with very few people needing much beyond the basic functionality. No one is doing amortization tables or complicated formulas. Word use is also fairly basic.
It seems that every week almost Microsoft changes the admin page for Office365 and adds or removes a feature or changes the way things look. Because of our basic Office needs, I'm seeing growing success with getting people to start and stay in the cloud. Prevents nonsense to a great degree. SharePoint to share between departments.
Oddly enough, I've had great success in "rescuing" Office files using LibreOffice running on Linux. Office files just add so much crufty proprietary stuff.