GNOME Boxes has a USB redirection feature that doesn't work in either Flatpak or Snap. Boxes has code to display a notice that USB redirection isn't supported on Flatpak when it detects that it's running in Flatpak.

I suggest instead adding a patch to comment out including devices_page in the preferences dialog for a box. I think this can be done in src/preferences/preferences-window.vala

The upstream Boxes maintainer thinks that there will be a new Devices Portal (included in xdg-desktop-portal-gnome) in time for GNOME 44. Once that's in place, both the Snap and Flatpak versions of Boxes should support the USB redirection feature.