I'm a powerlifter who has explored kettlebell training while the gyms are closed. I'm a total novice with KBs and have much to learn, though as a beginner I think I am ahead of the curve due to my background. That said, I am here because of two individuals' respect for Pavel:
1. Louie Simmons
2. Donnie Thompson
Everyone knows Louie has tremendous respect for Pavel, but how many know that Donnie has credited Pavel with a huge part of his world record success? Even recently, Donnie said on his Instagram that if he didn't discover Pavel's books/methods back in 2003, he would have ended up just a state record holder that nobody heard of. At 45 years old, Donnie broke the ATWRs for biggest squat (1265) and total (3000) in history. Yet I think he is just as famous for healing and keeping himself healthy, and is still fully functional and lifting heavy today at ~55.
At my current powerlifting gym there are no dumbbells, just fatbells and KBs. I was not in love with this, but I overlooked it because everything else about the gym and setup was A+. However, to my surprise my opinion changed quite quickly, and I now view them as critical to my training. Why? Two reasons:
1. Fatbells feel superior everywhere...wrists, shoulders, every joint basically. By superior I specifically mean there are no parts of my body that feel pain like they otherwise would with equal difficulty dumbbells. So when Donnie talks about less shear due to every infinitesimal point being equidistant from the center, in my experience this is absolutely true. Also, after the gyms closed and I had no equipment, I've resorted to borrowed dumbbells, and right away I noticed pain in my wrists with 20lb dumbbells that I didn't have with 97lb fatbells in my hand. So it just reinforced what I've known for a long time now.
2. Fatbells are harder than dumbbells...somewhere around 10-15% more difficult for the same weight/exercise. So you have to use lighter weight. So I look at it like box squatting vs squatting, where the former is harder and more fruitful despite the lower weight. So the training effect from fatbells is the same...less stress, less load, yet better results compared to dumbbells.
Another big bonus from fatbells is the ability to use them as banded kettlebells on the bandbell/rhinoflex bars. In my experience, this is the most common use of kettlebells in powerlifting gyms (a great use, but sad since they can be used for so much more directly in powerlifting, as Donnie has shown).
As I said in the beginning, outside of tying them with bands to bandbell bars for powerlifting, I am a total novice on KBs. I have just started doing KB cleans and snatches in my backyard, learning from Pavel's book and videos. I have no doubt that fatbells may be sub-optimal for certain kettlebell movements. Someone mentioned that it won't work your grip like a KB, I can foresee that. However, as a powerlifter, I get plenty of grip work, perhaps too much, since I am a hook gripper anyway (I'm sure many here know the last thing a hooker needs to worry about is their grip).
So to summarize, I view fatbells as a vastly superior dumbbell replacement, with the optionality of many (although not all perfect) kettlebell movements. I don't view them as true kettlebell replacements, and given that Donnie still trains with kettlebells, I doubt he does either. But I can assure you these are anything but a gimmick or fad.