@stokes the biggest downsides for me:
Since mastodon works strictly by follow relationships, you have to follow a lot and often to keep your feed lively, else you have to use relays to geta firehose of stuff to your instance.
The other is the need for forever moderation. Nobody's there to block all the CSAM and nazis for you, so you have to keep up with other admins and their blocks. There's a higher chance of you missing some asshole instance and getting harassed before you can have the chance to block them.