The biggest problem is that different instructors have very different methods and ideas about what is right and wrong, often damning what other instructors say as a load of rubbish when each method may well be perfectly safe.
When I fly with a student who has flown with another instructor, I let them stick to their standard procedures as long as they are safe. The key areas are things like use of carb heat, and touch and go technique (to put on power first or raise flap). By hopping from one instructor to another early on you get told contrasting things each lesson, and are too inexperienced to make your own mind up about what is the right thing.