Mastodon has many MANY MANY issues.
The first is that instance operators regularly abuse their users as hostages in personal petty fights. I don't care too much about drama, but there has been a lot of it regarding Israel/Palestine or Ukraine/Russia and instances defederating from each other as a result of said drama.
The second one is instances can go down for whatever reason - the admins just being unable/unwilling to cope with moderation, running out of money, getting into trouble with the legal system, ... - and users can't move their post, DM and media history to another instance.
And the third one is it takes them forever to ship updates. Bluesky is so much faster moving when it comes to implementing new features, but Mastodon ships even slower than Twitter which is an "achievement" in itself.