Let us entertain the possibility that there are flaws in the teachings of any given darksider.
Do these lead to the failures of students over the years? Of course. But it raises the question of whether the origin of these failures is in the lectures and practices espoused by one adept or another... or if it is in the student that fails to advance. For some, in having watched so many crumble over the years, it does leave room to wonder. Not for the person reading this though: The teachings presented and offered at any given time by any given practitioner are always flawed, without fail.
The straightforward teachings are only ever the best a practitioner can offer at the time they're given. But being the best an individual can furnish, at one point in time, does not make them perfect. Some of the more brilliant, deceptive teachings are deserving of praise because of the flaws intentionally worked into them.
They are flawed, and many have crumbled, or remained content with mediocrity. But that does not eclipse the reality of others having succeeded with the same materials, the same lectures, the same way of life. The weak and malcontent, the failures and the mediocre, will always outnumber the strong and victorious. The ratio of successful adepts to failed students is simply a reflection of that. However it is spun - that they were different, that their path led elsewhere, the dark wasn't for them - the bottom line is that some people are cut out for darkness, and for success. Others, are not.
There are always going to be flaws, no matter what is taught. They are there to be worked through, lived with, for those that are able and willing. They serve a variety of purposes, one of which will be explained...
"We are all born weak; strength is something we learn. I believe all weakness can be made strength." ~Luciana, Dark Knight
A competent Master will not drive away all but the best, at least not without providing the weak with initial opportunities to become strong. In this, the flaws serve students, forcing them to take what they have and work through its contradictions, shortcomings, and deceptions through their own application and experiences. This process in and of itself begins the process of culling the weak from would-be adepts. The aspirant will attempt to put it to use, fail, be confronted with the price of practice, and they will either turn away, deciding that the way of the dark is flawed... or they will stay with it, growing and evolving. Budding, unfurling. Becoming.
In relation to this, the failure of Dark Masters in the past has been their short-sightedness. With the idea of culling the weak and weeding out the unworthy, they approached newcomers early on, with no thought to timing, or to how the weak can be put to use for the advantage and growth of students that will stick around, that can hack it. Those that are too weak to endure or advance will be found out in time.
Dark Masters and Knights perceptive and experienced enough to work smart, rather than just working hard or with wasted intensity, won't flush their time or efforts down the toilet. Instead, the weak are allowed to be weak, providing a backdrop for the strong to stand out from as well as a resource for them to exploit, and the mindful, seasoned Darksider will guide his own attentions to enabling the strong to become stronger.
The lectures, however flawed, will always have a certain essence, a sense of darkness pervades, and they act as gateways. Even the most flawed will appeal to someone, they will act as starting points, gateways, pulling them in with allegedly effective methods to get what they're looking for, and forcing them to either grow or fall by the wayside as they go deeper down the rabbit hole. Because you will never know how practical, effective, or valuable any of it is, in any given lecture, technique, or exercise, unless it is worked with, refined, or discarded, on the basis of trial and error.
Ask yourself how flawed this lecture is.