I haven't instructed for many years but I used to. It was probably the most satisfying part of my flying career, so far, and I wanted my students to be the best possible pilots, which I think was the common position.
In the PPL market the individual instructor can have a significant impact but he is not in control, and much less so, since JAR-FCL. PPL students (in the main) are self-selecting. The only aptitude they have is the desire to learn to fly and (hopefully) the budget to do it. There is a disincentive for the school and instructor to be too heavy about the effort needed when a prospect arrived at the flying school. They very often do not have an ideal training schedule and I recall that some did not present themselves optimally. Often they were stressed or fatigued by some other aspect of their life and they had just driven around the M25 to get to the airfield as well.
I still recall one or two students who made very slow/zero progress. The instructors used to spend a lot of time worrying about them and how to deal with them. People do progress at a different rate, for a variety of reasons. Some did eventually progress to a PPL (after a large number of hours). Some went elsewhere, got their licences and then crashed damaged aircraft. When students did transfer, we were happy to send copies of our training notes.
The authorities designate the syllabus not the instructor, nor the flying school. Helicopter training is expensive, so you would have to be very persuasive to persuade most students to do much more than the minimum. A weakness in the UK system was that the instructor (and flying school) got paid by the flying hour: I guess it is still the same. I recall doing extra ground school when I thought it sensible and the opportunity arose. However, I also recall that some students just glazed over and you were wasting your time.
As some have already said, the PPL syllabus (indeed any flying training syllabus that I have come across) does not actually spend much or indeed any time addressing the main factors that cause accidents. These are not the ability to control the aircraft, but things such as attitude, character and decision making. As those of us who have been involved in CRM know, the people who most need that sort of education are the ones least likely to accept it.
Pre-JAR, there were some real problems with the "system". The one that I recall was that once private owners had acquired their licence, they did not need to EVER fly with an instructor or examiner again. We used to try and persuade them to do continuation training. When you succeeded, you were made well aware why they were reluctant - a number were so slow and hesitant at anything out of the normal (e.g. entering auto) that a real engine failure would have had a most terminal outcome.
As those of us who have taken our careers beyond the PPL know, practice and currency are very important at maintaining, let alone improving standards. PPL (post their licence training) will frequently be or be close to being out of practice. Whose fault is that?
Mind you, there are/were some flying schools whose pads and manoeuvering areas were a disgrace. Any attempt to raise concerns would be futile, until after the accident (maybe).