Self checkouts that just let you scan items without issue and accept payment are a nice enough idea for a bag or less of shopping, my problem with them is how they are implemented in reality (in Australia anyway). The first implementations I encountered I considered an useful addition but both the machines and the staffing changes due to them have steadily gone downhill in terms of user experience.

Instead of a quick painless experience you get a horribly touchy weight sensor which can't reliably handle particularly small items, particularly large items, or non-standard bags (and there are no longer standard bags due to plastic bag bans), a machine which demands assistant intervention at the slightest issue (and the assistants are understaffed so never arrive quickly), and when you finally get to payment it makes you click through an annoyingly slow interface to tell it you don't have a rewards card and don't care to donate to some charity before it will activate the card reader. To make things worse the manned checkouts are never staffed at a level - if any are even open - to cater for people with full trolleys so these end up clogging up the self checkouts (which have tiny bagging areas and are not intended to handle a trolley load) and making everything slower.

The icing on the cake is the self checkout treating you like a thief and throwing errors if the camera system thinks you didn't scan something in the trolley or letting off an alarm like you're trying to make off with something when you just want to buy a can of paint.