No real security impact for unprivileged containers, rather disastrous security impacts for privileged containers (it allows bypassing apparmor and accessing /proc and /sys in unsafe ways).
It’s not a real solution but it’s a working solution. The real solution would be to have apparmor’s parser not be horribly broken when it comes to mount handling… we’ve been reporting issues upstream for a number of years (look for mount in their bug tracker), a bunch of those are effectively security issues (some are private because of that). We wish the apparmor project would dedicate more time to fixing those, extending the mount handling and more actively test this feature, but there’s only so much that as users, we can do.