Two examples:
when you're browsing your follows on mastodon, and click on their follows, the list is not true to their follows (because your server hasn't fetched them).
and, when you first subscribe to someone's posts, you can't see older posts (say they've got 100, but you see zero).
I'm aware that there are technical reasons (you weren't subscribed), and open source reasons (nobody has the time to volunteer to fix it), but these are insufficient to help an anxious new user who's undecided about the platform.
That's only Mastodon, which has 7 years of refinement. Don't get me started on the litany of federation-related edge cases of Lemmy's UX failings.