One thing I’m observing when starting from a migrated profile (deb -> snap) is the following:
Certain content types might have a default handling program associated that worked in the past, e.g. snap:// would have been associated the snap store/ubuntu software/gnome software application. Now, in the snap confined world, such direct calls will not work and just silently fail (you get something like “env: ‘snap://app’: No such file or directory” on the console).
In the snap, you should likely always use xdg-open or the portal in general. However, the average user might not discover that they can reset the association in the settings and will be frustrated by the lack of feedback.
I think those content to app associations must be reset on migration (or amended).