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.

Facetime too. I shared a laptop with my wife for like 2 years, so it was an ok experience, but we noticed those little things.

I wonder if this was a design choice, so if I’m on the computer and a call comes in for them, I can let them know and maybe hand it off?

The alternative would be they would have to answer on their phone (assuming they have an iPhone, which may not always be the case), then use handoff to get it on the Mac.

Could be, but I've never wanted that. I just answer it on my iPhone or my desktop Mac.

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.

Yep on ChromeOS each user's home dir is separately encrypted with their own password.

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.

For me quitting preview, or maybe it is settings, resolves it.

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.

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.

Non-admins getting prompts for system and app upgrades is mildly annoying. The bigger one in a family setting is the clunky sharing. There's no good way to share a photo library or music library between users. The Unix version of making a folder shared by a group doesn't usually work for Apple apps.