I worked in a barn that had Nelsons. Overall I liked them, but there were plusses and minuses.
Plusses-- not hauling water. Easy to clean. Nice looking. 99% of the horses used them with no issues even if they had never used a waterer before. Water was always fresh.
The major drawback was if you have a horse that dunks feed/hay or just gets dirt in the waterer, it can work its way under the bowl. That led to some pretty slimy/nasty waterers and cleaning them was somewhat difficult. I was always worried it might gunk up the insides enough to cause a fire, though I never saw that happen. They sell a grill that can go over the bowl. It sucks.
The other thing is maintenance. There are no “Nelson” service people. You just have to call a regular plumber. We had… very varying success. Some could figure it out, some had no idea. Nelson was completely unhelpful, wouldn’t send an owner’s manual, wouldn’t talk to the plumber. You are on your own once you get these. We had one waterer where the scale somehow got out of alignment so the bowl didn’t fill when it should have. It took 3 plumbers to get it figured out.
And then the major thing I worry about is what happens when the electricity goes out in the winter. Does the whole things freeze and break? Again, this never happened-- but I worry it could.