At my local Fred Meyer, which is owned by Kroger, it's 6 self-check per employee. It moves way more volume than a single cashier scanning everything would.
But more importantly, the customer is doing something most of the time. They're not just standing there waiting. So it's a lot less unpleasant.
If you don't scan an item, or if you try to put the wrong item in the bag, the AI watching from overhead camera will stop the process and flash a light until the clerk comes over, and then it shows a video of what happened and the human decides if it was a mistake or not. When they first installed the AI, it was going off constantly thinking the advertisement on the back of the cart was an unscanned item. But they fixed that within a couple weeks. I did set it off one time by picking up two of an item, scanning the first one, and then putting it in the bag. It noticed I picked up two and scanned one, but not that I was still holding one. So it does slow me down slightly.
A funny thing happened, I scanned some sake and was waiting for the clerk to do the ID check, when a person from the customer service desk was walking past on their way back from break. Soda bottle in hand. They stopped to help out, and did the ID check, but as they walked off the AI alarm went off. Then a second person from the Customer Service desk saw what was happening and came over to see what he messed up; the AI caught the employee running off with an unscanned soda! It was pretty funny.
There is almost no theft that takes place at the self checkout now. The AI isn't perfect, but you're not going to sneak an item into the bag without scanning it.
But thank you for standing in the other line, it only takes a few really slow people to gum up the works.