It's been our experience that dungeons which are pretty challenging for the players turn to cakewalks once that first gold chest is opened. The sheer firepowers and defensive capabilities that pop out of it turn a hard fight to a one-shot kill on the boss.
I've also heard people say that RtL starts off hard and ends off easy for the players. How much of this is due to the gold treasures?

, so I don't think it is as bad as it looks. If you play with the official rules for Treachery however, it might be a little harsh... Although I can't really know since we haven't tried.