I believe HST and 5x5 (and DC and volume training, etc..) do work. Unfortunately I also believe 150% that they do not work for the entire body. You may have certain muscle groups that respond to that kind of stimilus and training and then have other bodyparts that simply don't.
I don't believe a training system exists that works your body as a whole with the same level of benefits. It truly comes down to a case of the sum of the parts being greater than the whole.
When trying other training systems it is crucial that you first stick to it long enough to effectively judge how well it works. This means 5-6 months at least. And also keep a training log that details every little thing you can think of. Then when try another method, you do the same... and again.. until you can look back and compare notes and find out exactly what you need to do for each.
In my case, I do around 8-12 sets total for chest. Hamstrings get about 12-15. And triceps get around 18-25 sets. However, the way I train each is different. Chest I use heavy weight to break the muscle down and fatigue it. With Hamstrings I use a slightly higher rep range. With triceps, I simply pump blood in there over and over again. The blood volume training principle.
The reason most people don't see the results they want is because they have become ingrained with their training and training concepts. The 5x5, the DC, the Secret Soviet System (remember that one? LOL) all have the same thing in common. Heavy weights and low reps. That is why some people gravitate from one of these to the other and vice versa. That is what "worked" for them once and they subconsciously stick with it.
Which isn't to say I haven't made the same mistake. I have. When I first started training right out of college I went to Lee Haneys Animal Kingdom. I had never touched a weight before. Instead we just did training drills, abs 6 days a week, and various pullups that our swim coach gave us. From doing this 8 years (4 years high, 4 years college) when I first walked in the gym, I already had low bodyfat, deep abs, shoulders and an above average back. Naturally I started training heavy. Reps in the 4-6 range. I made a lot of gains to start with the first year. And who doesn't in their first year of training? We all do. Then it slowed down and by the end of the second year it had stopped all together. So what did I do? Trained harder. More weight. More sets. Still low reps. It worked for me to start with, it should work for me now right? Well most people don't realize that any kind of training will work for them when they first start out. And I was trapped by my own notion of what "worked".
I had a conversation at the water fountain one day with Lee Haney that changed my mind and got me to trying new things. He came over and said "I have never seen a powerlifter with that little amount of bodyfat on them." I told him I wasn't a powerlifter. And he replied with a straight face "so why you training like one month after month after month?"
Come to find out, he didn't train with low reps anymore and didn't believe in them. He said anything less than 8 reps, you are fatiguing your tendons and joints first for having to compensate for such a heavy load.
It took me years to find out what works for me. But at least I did. That is more than some people do. Instead they train the same, and still look the same year in and year out.