This is, in my opinion, the most powerful card they've ever printed. This ruins mulligans and a lot of turn 1-2 plays, and often has the potential to give you actually useful hero powers (life tap comes to mind). One slot in your deck to ruin your opponent's mulligan is an absurd advantage. The only rogue decks that won't be running this are odd rogues, I guarantee it.
So what does that mean? You Just have a different hero power? and why is that good?
Your hero power and neutral cards won't reveal your true identity, so your opponent will play and, most importantly, mulligan around another class until you play a Rogue card.
Also worth noting discovery cards will give you cards from the class you're disguised as, minor benefit but could be good. Considering this only requires one card slot I wouldn't be surprised if this sees play assuming there's any kind of good rogue deck out there this xpack
I'm just surprised the rogue legendary wasnt Matthias shaw tho honestly
A: Depending on the meta, some to literally all of the played don't or rarely care about their opponent's class because they're hard mulling for the key cards they need for their combo and they wouldn't change their mulligan regardless of class
B: You can't choose your fake class, so you have to get "lucky" before your match even starts for it to "work", i.e, if you get hunter they'd mull for the same cards they would against aggro rogue so all you accomplished was including a bloodfen raptor into your deck over *insert good card here*