It’s safe as long as nobody breaks the rules. If law enforcement or copyright holders really wanted a login they could probably get an invite from somebody. The weak link in security is always going to be a person.
Once they infiltrated a private tracker, they’d either have to hack them or download every torrent they want to and track down the seeders. 90% of the seeders would probably be in a different country so, is it really worth their effort? They’re not going to get the people ripping the shows and movies.
So yes, I think getting caught on a private tracker is possible. Perhaps not probable.