If your like me, then you enjoy playing Descent. I am a very competitive person. But I had to realize that one of my campaign groups was just simply not ready for that. I didn't want to stomp them because I was afraid if I did they wouldn't want to play anymore. So I intentionally held back. It was hard for me to do, but I want everyone to have fun. And more importantly I want to keep being able to play.
However, the OL is going to win. Some quest here or there, an act, or maybe the hole campaign. Thats something the heroes just have to accept. Because other posters are right that the Overlord is not a GM. I GM pathfinder campaigns as well, and I will tell you its a completely different mind set. In Pathfinder, I am trying to create a fun challenging story for the heroes to eventually succeed. But that is not the case in Descent. The overlord player wants to play to win. I would just caution though to know your group. In the end you want to play, and people will just quit if they feel like they don't have a chance.