I've trained at a few different schools from small country strips to large city, and probably been exposed to around 15 different instructors , all different experience levels.
The one thing I can say about teaching is that some people have it, and some don't. I have flown with excellent pilots who are useless teachers I.e no skill at passing on their knowledge but by watching them fly you can learn alot ( backseating is a bloody awesome priviledge), and I've also flown with fresh stamped instructors who's passion and skill at handing over knowledge have been invaluable. The ones who sucked were the ones who were too selfish (or afraid) to actually let me make safe mistakes or the ones who obviously hated it. To do this frickn thing as a career you need a fair dose if passion, anyone who douses that with a crap vibe is doing the industry no favors.
I would also say that more often than not, the students unwillingness to accept their faults, or not do the non flying work is a massive hurdle to someone becoming a proficient pilot.