My biggest pet peeve with the self checkouts has always been their physical size.
If I have 70 items in a cart, the last thing I'm going to do is try to checkout at something the size of a voting booth with enough room to put, maybe, 2 or 3 full bags.
Well, just throw the full bags back into your cart! Yeah... my cart still has 62 other freaking items. Second, empty cart? Hell, there's not even enough room for the first cart at the checkout.
I walked out of a store some years ago, leaving a full cart behind, because there were no manned checkouts open (lots of room on those belts) to land stuff, and lots of room for them to stage your bags as they pack them) and the manager's attitude was basically "suck it up and/or *expletive deleted*ck off". So I told her I'd be *expletive deleted*ing off to her competition and she could deal with the cart full of stuff, including a bunch of perishable items.
The newly opened store near me has something new... belted self scan checkouts. They're the size of a regular checkout with the conveyor belt, which means you can unload your card wihthout a hassle. Much easier to use.
And, the Walmart near me also has something new -- a larger self checkout stand (no belt) but with a bunch of room on which to bag stuff once you've scanned it. No more trying to fit 70 items on a postage stamp.