We have discovered one more weird interaction that enabling parallel installs can have with existing snap mount namespaces, see Parallel-instances breaks content snap consumers for details.
With this in mind, I would recommend that if the users ran any snap applications in the currently booted system prior to enabling/disabling the feature, they should reboot after toggling the experimental flag state.
Alternatively they can discard the mount namespace of all snaps, though I see that as something that a snap developer or a snapd hacker would be more willing to try.