I've never done self checkout or curbside, anywhere. I don't have a reason to complain, and in fact believe those options/services have a place.
My "complaint" was the employees standing around monitoring the self checkout lanes. That is an example of inefficiency, which has a cost that is passed along to the end user (the customer), whether we use that option or not. It affects the price of everything. It's not just Walmart.
Have you noticed a savings by using the self checkout? Having never used one, I don't know. I also expect that I must use a credit or debit card in these lanes, which can have direct or indirect expenses associated with the transaction.
A business owner should be mindful of ways to increase efficiency for a lot of reasons, even if it's just to increase their profit. They should at least consider the waste and who pays for it when they have employees standing around having customers do the work for them.
Considering this further, maybe I can get my customers to work on their own sheet and pay me to watch. Genius. I'll only need to figure out a way to pass that idea along to the masses, so they believe it's a great deal.