This seems to be an implementation issue. In my neighbourhood discounter, in Germany, there’s three self-checkouts and while they’re a bit small they also don’t do any of that weighing and whatnot bullshit: You scan your stuff, pay, done. The only thing they can’t do is apply best-before rebates.
There’s also always a manned till open (or at the very least, when things are slow, a worker hanging out in the vicinity). In practice if the queue is empty you go there, if you have lots of stuff you go there (because it’s bound to be faster as you can focus on packing while things get scanned), otherwise you have the choice to use self-checkout. Never had to stand in line for self-checkout, before that happens they open another manned till. What the self-checkouts do is keep small purchases away from the manned tills when they’re busy which is exactly what they’re good for. I