I think the problem was that Troi was too close to people like Will, Beverly, and Jean-Luc to be an objective therapeutic influence, except in the most extreme circumstances, i.e. recovering from abduction, torture, brainwashing, etc... The rest of the time she was too much of a family member to affect the proper environment where she could counsel them. So, I'd say because of that, Guinan had more success as a counselor or adviser.
However, Troi was clearly the better therapist. Guinan didn't really supply therapy, and the counseling was minimal. More often than not, she was just a listener, a sounding board for the senior staff to externalize their inner conflict for the audience. In that sense, Guinan isn't a counselor at all, not in a professional sense. She certainly isn't right all the time