For myself, knowing what I know, when I've got an issue, I'll always go back to an intermediate groomer to sort myself out. Usually it takes about two runs doing a couple of things that work very well for 're-centering' myself. When I'm coaching, I mix it up. We'll do some groomed for setting some goals and presenting a couple of ideas and observations, then subsequent stuff will be used to address the same challenge/issue in varying terrain, both on and off piste. One idea, lots of mileage. I even use the runouts to get folks out of their own pre-existing head.
I always tell clients/kids/students who are advanced skiers that if I take you to easy terrain, I'm going to make it very challenging for you. My job is to 'un-bore' them and make more effective skiers. For people who are really willing to go to square one to make it happen, yes, groomers for working on transitions are very effective. Nail it there and off piste is cake, but it's much easier to sort out on piste. For most, I'd do a combination. Sometimes a bit of flail on inappropriate terrain makes a point better than words coming from my mouth. The trick is to use it in a limited way or shortish pitch so as not to completely dishearten someone. The point isn't to show them how good I am, but rather how difficult it can be to make changes in terrain that adds a completely different mental or 'fear' component.