Effectiveness of Mentzer's methods depends a lot on the individual and how they interpret "training to failure". One mans failure, is another's version of when the set really starts working. If its working for you at the moment keep with it until it stops, then go back to a more traditional conservative approach.
From my personal experience(when natural I haven't tried it yet on PEDs)it sounds great in theory but really led to overtraining and chronic injury in the long term.
As natural bodybuilder I responded better to more frequent workouts 2x/wk/ bodypart, with moderate to high volume, working the muscle most of the time a couple of reps short of failure. Multiple sets with this approach fatigued the muscle. Occasionally I would go to failure, or do drop sets but it was sporadic otherwise I overtrained, or got injuries.
I did a lot of Mentzer/Yates type workouts in the late 80's early 90's, over a long period of time it becomes really draining on the mind and the body. I was the type of guy who really should have reined the intensity in. Eventually psyching yourself up to go to failure and beyond on a consistent basis is too much for most natural and even many guys on gear, even if only one set per exercise. Form and posture go in the attempt to get the maximum effort and this leads to acute and then chronic joint problems, if not as well as systemic overtraining and exhaustion. For me personally I find it better to rely on form and technique, cycling poundages, multiple sets, consistency over the long run, rather than max effort for one balls out set.
Some of the other issues I have with Mentzer's theories, it was more like a marketing exercise to sell books/manuals, a religion of heavy duty. When you hear contemporaries talk about Mentzer training they say they saw him training multi set like everyone else. Yes it could be that all but the final set he did wasn't to failure, but it seems to over play that you only needed to do one set to failure.
A lot of the Nautilus stuff advocated one set to failure, where I guess Mentzer got the idea from. Casey Viator who did famous test where he put on 60lbs(from memory, could be wrong, but it was a lot), after using Nautilus program for a couple of months was a bunkum marketing exercise for Nautilus and their training methods. Casey was on gear before and had plenty of good mass. He went off the gear before the test and heavily dieted, to reduce his body weight to skew the results. As during the test it was most likely that he was back on the gear, and the results were mainly due to the return of heavy training, regaining his lost weight, not building brand new muscle.
Great marketing to convince the ignorant at the time(myself included)that brief exercise and Nautilus machines were all you needed.
Mike, Casey, Dorian were all taking gear, which at the time was at unacknowledged to the greater public. Lets face it if you are on substancial amounts of gear you are physically capable of doing a lot more in terms of weight lifted, exercise frequency and intensity, than if you aren't on the gear. Virtually any weighted exercise program will give impressive results.
Even Dorian Yates who successfully interpreted Mentzer's heavy duty system came to the conclusion that he couldn't train that way all the time, especially later in his career, especially in contest prep. Multiple tears and tendon ruptures shortened his career(a fine legacy though).