I see dogs that have lost the joy of working and are just going through the motions - because they have been drilled hour after hour. You should never work a dog to the point of where the dog sours. If you feel you have driven a long way and don't want to quit on a bad note then stop and take another lesson an hour later. That will keep the dog enthused about working which is so paramount in keeping the dog working to the best of his abilities. I don't think everyone is qualified to teach and I believe one should be very careful about whom they choose as a trainer.
Some people talk a good line but have never trained a dog to an Open level.
I will give an example of possible repercussions from an owner taking lessons from a person that hasn't trained a skilled Open dog. Let's say a student has a dog running well in Pro-Novice and is attempting to progress to Open. Eventually he does accomplish this goal only to find his dog does not have enough push to finish an Open course. Not only are Pro-Novice courses smaller but they also use dog broken sheep so often a non pushy dog looks perfect. This is only one small example - there are many more.