They also fuck up because they aren't designed and implemented properly.

• Walmart's don't accept tap to pay.
• Whole foods' requires manual keying in of pastry items as different options (they don't have danishes in their DB so they need to be rung up as a bagel; per the human worker that resolved the issue for me when I predictably couldn't find the item they failed to include).
• None of them allow you to cancel the order (such as when you want to check the price of an item because the store neglected to actually list the price on the floor).
• None of them let you remove an item (such as a duplicate scan or removing a luxury item that stretches your budget or rang up higher than you were expecting).
• You can't purchase shaving goods, alcohol, canned air, or other adult items without intervention (probably no way to actually avoid this one, but it doesn't promote a smooth flow) and the kiosk often locks down until aided by an associate preventing you from continuing to scan your items while you wait.
• Often locks the kiosk when placing a reusable bag in the bagging area.
• Inconsistent payment methods: some allow you to scan your card at any point in the process, some process payment the moment your card is scanned, some require a manual trigger on screen prior to scanning your card.
• Often forces popups between scans ("This kiosk is in card only mode," "Enter your loyalty card number," or "how many bags did you use today?")

I'd like to:

• Walk up and set down my bag
• Scan all my items
• Remove arbitrary items
• Tap my card
• If required; verify my age and have an associate clear any blocks
• Grab my stuff and leave

Instead what often happens:

• Walk up and set down my bag
• Kiosk locks because there's an item in the bagging area
• Pickup my bag, move to a different kiosk and set my bag on the floor
• Scan my first item
• Dismiss the card only pop-up
• Dismiss the loyalty pop-up
• Scan the item again because the first scan just wakes the machine and the order doesn't start until you dismiss 2 popups
• Put the item in my bag on the floor
• Scan the next item
• Dismiss pop-up about first item not being in the bagging area
• Take first item from my bag on the floor and set it in the bagging area
• Kiosk locks until associate clears it
• Scan a razor blade
• Kiosk locks until associate clears it
• Scan the remainder of my items
• One of them scanned twice
• Click the visible delete button next to duplicate item
• Kiosk locks until associate clears it
• Tap my card
• Realize that this unit works differently than the last one I used and click the "Finish and pay" button
• Select card as the payment type (on the kiosk in the card only queue getting run in card only mode)
• Dismiss the bags used pop-up
• Tap my card again
• Move all my items from the bagging area to the reusable bag on the floor
• Collect my receipt and goods and leave

I'm glad that you've consistently had a good experience with them, but I have not. While each of our experiences are anecdotal, the machines' failure to routinely accommodate my expected use case is an engineering failure. I am a software engineer by trade and know how to interact with computers well. While we have a running joke about customers not reading what's on their screen that's no excuse to design an interface that cannot properly react to unexpected or unusual inputs or tasks.

There's a certain store I go to that needs an employee almost every single time because the scales are insanely sensitive and lock you out immediately if they think it's wrong.

The self checkouts where I am get confused from things as simple as a customer placing their own bags on the scale before even scanning anything and constantly need staff intervention. Not to mention how often prices are wrong on these systems. For the cost of constantly developing, upgrading and maintaining them. In the long run companies would be better off training a few extra staff for express lanes instead. Only my humble opinion though.

The scales are always off and unnecessary anyway. They factor shrinkage into the price.