As a very simple example, airdrop to macOS with multiple logged in users will frequently pop up the confirmation notification in the user account that is not active.

Perhaps I don't understand it but the encryption security model for MacOS/iPadOS/iOS currently doesn't allow multiple different encryption keys for each user. So any user can decrypt the whole drive and while it does enforce user permissions, the security model can't support true multiuser.

I actually don't know if Windows or ChromeOS support this either but this is certainly something Linux can with LUKS et. al.

As soon as I added a 2nd user, my Samba share totally broke and days later I still don't have it working. It was fine for over a year and now I'm close to deleting my 2nd user just so I can access my Mac Mini across the network again.

USB security prompt disappears when multiple MacOS accounts signed in

Still a problem for me, and has been for years, but I may be holding it wrong. https://discussions.apple.com/thread/255929514?sortBy=rank

The solution posted in the discussion is not really secure.

Switching users while changing displays often results in an incorrect resolution. That’s such a basic thing: different users have different preferences for their displays and keyboards attached to the displays. Yet this doesn’t work reliably, as if during some moments the login window just doesn’t want to adjust resolutions.