Because I suspect that snap uses both /home/user/snap and /var/.../snap, and needs them to be on the same disk (to allow hardlinking, I suppose).

Have you tried it?

Sounds like something a bindmount might fix.

Back in the olden days before simple and convenient packaging systems like snap had been invented, we had to get pretty creative so that stuff didn't freak out because it was split across filesystems.

I haven't tried it, but I can remember people on forums having mixed results with a bindmount. I guess there is no guarantee that it works until Snap has been running for a while without problems.