I assume admin/moderation issues are a given, so uptime/reliability. I don't have issue with my instance, but I see a lot of image posts on another instance that I want to view and they regularly don't load until I try again much later (even opening the URL of the original instance in a browser doesn't load, so I don't think it's a federation issue/issue with my instance). I don't want to name the other instance as I don't like to bad mouth what are almost certainly volunteers, but I do wonder why people haven't abandoned posting images to it due to reliability. Maybe the posters aren't aware.