The question is basically the title.
It’s very hit-and-miss. But mostly miss. That picture of a cat hiding under wrapping paper taunts me.
It doesn’t seem to be related to what instance the content is on. But I haven’t been able to find any pattern.
Is there an alternative (for non-instance-admins) that works more reliably?
I think the main reason for unreliability is admins defederating the instance it gathers data from. For example all piefed instances block it by default.
Not sure of the exact rules, but it might be something like it won’t get data from any post or comment by a user on one of those instances.