“Shortcuts” are wonderful if effectively and appropriately applied by someone who understands what’s being “short-cutted” and why…and whether the short cut will end up in a (widely ineffective) “bandaid,” or a true “fix”/improvement. I find many shortcuts to be preferable to an application of “sweat and tears,” both of which are far too drying on leather. 
The Pessoa rig is a way to help develop topline (and bottom-line/abdominal) muscles in a horse without the need of a rider (or the need for the horse to compensate for the rider). I wouldn’t consider it a “gimmick” or a “bandaid fix” in any way if used appropriately.
But I find that rigs like that, the ones that take an effort to set-up and then use on a regular basis, are best used in the hands of someone who wants to spend that time off of the horse. I am terrible about my daily riding schedule and what I want to get done. I prefer to do as much as possible under saddle because it fits into my routine so much easier. It might just be perception, but riding 5 horses in a row is easier for me to mentally manage than 4 rides and one lunge.
So in the name of gaining similar benefits without the need to lunge, I love my Equiband system (I can’t remember the website offhand since it’s the company name and not the Equiband name, but should be easy to google - and probably would have taken less time to google than to type out that you should do that, lol). The system is a saddle pad with buckles sewn to it that you attach 2 resistance bands to (one behind the butt, one under the stomach). I’ve used mine on multiple horses and felt that it enhanced my conditioning program. But you could also buy resistance bands (or use polo wraps) and tie them to the saddle to get the same effect much less expensively…though from a long-term approach, the actual system is much easier/better.
And I just have to say that the word “shortcut” used in a derogatory manner is one of my pet peeves among horse people. The image so many people seem to want to claim, “I ride all of my horses, without ever lungeing, in a plain snaffle, and no extra tack whatsoever because I don’t believe in shortcuts” is just silly. As is claiming that every tool is a “gimmick.” The pessoa rig is a “tool” just like the myriad of bits out there, the variety of martingales, reins, draw reins, neck stretchers, etc. Every tool has a purpose and a true horseman makes an effort to understand where those tools might help a horse reach a breakthrough or speed up (in a positive way) a lesson. Yes, the bad horsemen often rely on those tools to make something happen, but it’s not the tool’s fault to be in the hands of a shit horseman. And yes, many of the tools require someone to be a better rider than most people who would want to use them are (e.g. a harsh bit or draw reins). But a tool is a tool is a tool, and it’s up to the artist to make it something that creates or something that destroys.
