One downside/senario I'm worried about is what happens when something really bad happens. Like illegal authorities-get-involved bad? Like leaking sensitive government information or a homegrown r/ jailbait situation that the media catches wind of. Stuff can't be permanently deleted, at least not without nuking everything around it...which people might be tempted to do. And that's basically turning anything seriously incriminating into essentially an infohazard that could get you nuked because you're in an instance where someone else from it commented on the thing or something. And any attempt to and defederation from the offending parties probably isn't the hard shutoff that the authorities would be demanding in such a situation. Even if nothing effectively happens to the greater Federation, it would be a PR nightmare that would probably kill any future attempts of evangelising the platform in the future, especially to bigger communities looking for a new place to stay.

Places like Reddit have mods and admins that worst case scenario, can be the scapegoats. Lemmy doesn't really have that layer of protection because of how esoteric it is to the layman.