Rule number 1 of third party programs: It is NEVER safe.
With that said, there's never been a ban with PoeHUD, as far as I'm aware. Does this mean it's safe or ban-proof? No. They most likely could ban everyone using this by pressing a button or something as easy as that. Will they do it? I don't know, and no one does.
So, for the not detectable part: We don't know, but probably is if they wanted to. Will they do it? So far, they haven't, who's to say they won't