The latest version of Lemmy is not stable. I don't know what the issues are, but I keep hearing about federation issues in version 19 + . I know lemmy.ml had defederated from nsfw instances, including yiffit, but not pawb. Pawb is much more proactive in defederating compared to yiffit. I can't find it, so from memory, but because federation (including "all") is an active process and doesn't happen automatically, yiffit is much more, "if there is no interaction, there is no need to defederate". You can see who a parcular instace has defederated from by going to /instances on that instance.
As far as stability, although not constant posters, the admin here, and those at pawb usually respond to pings and are active on mastodon as well. So long as the server itself is running, you can check https://status.yiffit.net/. The only downtime I can remember here was a few hours one day. Ironically the day somebody posted congrats on the uptime.
I think there are some tools to help move an account across instances. The upload issue is not only on feddit.uk