Yes, I've beaten the high intensity horse quite a bit, but there is still a lot of confusion out there about it, so I'm going to keep beating. I've also seen quite a few videos (like when I check the channels of my subscribers) regarding "changing things up" and "trying something new and radical" in order to get the results that haven't been achieved using other methods. There are a few issues that will prevent the "shaking it up" from working super well in most cases and I'll talk about those in a minute. But ultimately, HIT absolutely works! If it's not working, you're not doing it right! There's a reason most huge bodybuilders gravitate towards the same type of training in the end--if you look at the splits, rep ranges, and general training principles being used by most pros, you will see that they are largely the same.
Firstly, the whole "surprise the body" thing just isn't real. And I'm not talking about surprising it with explosivity, I'm talking about never doing the same thing twice in a workout and never doing the same workout ever again. Your body adapts to repetitive tasks, not sporadic ones. Surprising your body just means applying progressive resistance and the associated adaptation demands only once, and expecting results. It doesn't work that way. Understanding that the body responds slowly to training may help explain this: the results you're seeing this week are completely unrelated to that same week's training, in all likelihood, your body is several weeks "behind you." I talked about this, and the related problem of "PR's" here and here.
Secondly, specificity plays a large role in the mechanics of hypertrophy. Your body responds to specific demands; i.e. you get huge legs from leg training, but squatting isn't going to help your biceps peak. This negates sporadic training styles as well. In order to fully activate and fully exhaust the muscle in question, something more than a brief set of kettlebells is required. In order to develop that muscle fully--that is, to proper size and proportions--it is necessary to use varying weights, angles and strength curves, i.e. different exercises. Additionally, the type of training associated with HIT is specifically designed to maximize the benefits of that training--proper form, intensity, and mind-muscle connection all contribute significantly to gains, but they are not mutually exclusive--they are all equally necessary. These are harder to acheive, and combine, when it comes to circuits, interval training, and other "sporadic" training styles--if you never do something twice, how will you ever do it well?
Thirdly, the most common tendency when "shaking things up," is to go way over-board on the radical side of training styles. Although 12 sets to failure might make you feel like you're doing a lot of hard work, in reality, you are only training your body for endurance--which will be evidenced by a lack of size from this style of training (but on the other hand, great endurance). Going the other direction, and training retardo-heavy, will only result in injury, nominal gains (see my post on the idiocy of PR's), and at worst, chronic overtraining symptoms such as tendinitis and tendinosus. Once again, proper form is absolutely necessary if you want size--and you can't do that when you're training too heavy. Your intensity level should be high enough to preclude additional failure sets as well.
The problems most people have with HIT training can be summed up in a few ways: they enjoy training, and they don't want to do only one all-out set per exercise, or only a few exercises altogether. Or they're convinced they can do another set heavier, so they pyramid for four or five sets--every workout! Then there's the people who just don't understand intensity. They are not pushing themselves, mentally, to connect with their bodies, or pushing themselves farther than the week before. And then there's the people who don't really have a split at all, they just overtrain their favorite bodyparts constantly and neglect others *cough*legs*cough*. These all result in people getting stuck in perpetual plateaus, getting frustrated because they're not seeing the results they want, and changing things every few weeks out of impatience. HIT works, but you have to use all the principles properly. Just commit to it and then give it time. Once again, HIT is one all-out set per exericise, 6-10 reps on your own (plus cheats, partials, negatives, or forced depending on the exericise and other limiting factors), 3-4 exercises per bodypart. Get in, get out, get huge!
In the end, if you want to be large and in charge, there's only one way to do things. The right way, from the beginning, all the time. Yes, it sucks being on a diet year-round. Yes, it can suck training legs when you really want to hit upper body. Yes, it sucks going home when you want to keep training. Yes, it can suck when you can;'t drink a lot on Friday because you know you are trianing the next day. You have to decide if all that work is worth it, and if it isn't then you need to stop bitching about not looking the way you want to look. You can't have your cake and eat it too. Or in my case, you can't have cake!