If you are visiting a .onion site there is no exit node involved. Traffic is encrypted and inside tor all the way from you to the server hosting the hidden service ... usually you need to have some trust in the operator of the service anyway. Here for instance that they don't log all the steem-passwords ... if you trust them not log your steem-password you can probably trust them not to run attacks against your connection to try and determine your location also....
RE: UnderSteem - Steemit without censorship (NSFW filter removed) (via TOR)