The epitome of the term compound movement. Often overlooked and/or dismissed as “some funky kettlebell exercise”. I refer to it as “weighted yoga” because you can’t/shouldn’t speed through a rep and you can take your time drilling each transitional pose. One full rep on one side might take a solid minute itself.
I wouldn’t expect them to really stimulate hypertrophy exactly because they disperse tension throughout so many muscles without really overloading any particular muscle group.
Really the only muscles that are going through significant ROM are the legs, whether you use a lunge style or squat style, but the load isn’t anything that’s significantly challenging to the legs. Pretty much everything else is working statically, which isn’t great for hypertrophy.
I actually learned about it in one of the first T Nation articles I ever read: Exercises You've Never Tried 1
And yeah, because so many bodyparts are involved, it works well as a diagnostic to find weaknesses. But shoulder mobility/stability will pretty much always be the first thing to notice.
I think Dan John has a test about doing a tgu with a cup of water, the goal being… don’t spill it. I haven’t tried it yet, but that’s definitely one way to make sure the arm stays vertical and doesn’t drift.
Not sure if you mean how much weight or how much like how often. How much weight, as I explained a while back, I use 185 pounds on everything. 
For how often, it’s sporadic. More due to attention span and/or focus than anything related to the exercise itself.
I haven’t checked, but I’d be super-surprised if they did. It’s a very, very niche exercise and I wouldn’t expect the majority of researchers to know what it is, let alone put major time into investigating it.
On par with most other overhead exercises and/or deep squats. Risk of dropping the weight on yourself, risk of the shoulder going a wonky way, risk of some ab/low back stress during the first transition up and on the descent, risk of some hip issues coming up from the bottom.