To the original poster the first problem with the question of "What constitutes success?"
Everybody keeps looking for an easy way out, and right now there is no easy way. Programs based on manual manipulation techniques which include stretching and all the rest take many years of dedicated work to be effective and this is variable by the individuals.
Surgery is effective in a small percentage of individuals and according to statistics the FDA and several other major medical authorities the chances for long term damage or damage delayed requiring corrective procedures later is very high. It is for that reason that the FDA has never approved PE procedures.
The one place I do agree is that documentation on successes is extremely poor at best and this is because organized medicine would have to both test and approve of something that: A.) They did not invent and can't take credit for and B.) As a result of A it is not unlike the scene from E.T. when the little alien is in the kids closet posing as a toy to avoid detection, it can in fact be right in front of their face and if they cannot take credit or the profit they will deny it's existence.
There is risk with everything you do starting with getting up each and every morning. Odds are against one being hit by a falling piece of space debris in their living room or a falling satellite, but because junk falls from space every day it is possible.
Are there injuries from PE techniques, that is an absolute yes. However the thing that appears amazing is that compared to the surgical procedures which have gone so very wrong with MD's using failed techniques again and again, PE by manual manipulation and stretching appears to have a far superior safety record. You don't hear of many injuries with this in spite of the number of people doing it.
The sad part is that everything I have said in this has been said in 200 other places before and by more than just myself. There is under PE in this group threads on nearly every form of PE and a few code words in the search function will take one right to them. At the same time use of search functions will take very little time.