Yeah, honestly, I was fine with the way backstab was handled back in the day with just thieves (and assassins) being able to do it. I wonder how much this whole elevation/backstab biz is just bleedover from div2? I can see surprise giving advantage, sure, and having a bonus for surprise when concealed or even at elevation by way of concealment but having advantage last throughout combat has broken a lot of fights in BG3 and previously.