Most of the issues responsible for the shut down of all but the one horse slaughter operations in the USA had everything to do with the way it was being performed.
There were guidelines in place outlining the methods that were to be used, but instead of adhering to them, many many companies cheaped out and did it abhorably.
Instead of a one on one bolt gun operation, they would gather them in large masses and do them together. So the horses could see and smell and hear every other horse in the room being killed before it was their turn. This was not as safe for the workers either, but of course, it was cheaper.
The difference between horse slaughter and cow slaughter is that horses have much more highly developed senses and fight or flight response. Enough so, that it makes any method of killing horses in the same place where other horses have been/are being killed, completely inhumane and unsafe. That's just the way they are and the only way for it to be humane is with a gun in their own paddock. But of course that would have been too unprofitable for the companies to stomach.
If people have to put down their own horses on their own property to use the meat... well at least there is an enormous likelihood it is more humane than any commercial operation. I'm not going to cry for the people who make that decision, LOL.