It's like self-service checkouts: the store gets to save on stuff and get their customers do the store's work instead, and we don't even get a discount for that free labor and more time spent in checkout and degraded experience.
"Consistency" and "predictability" are not the words I associate with self-service checkout. Maybe I'm spectacularly unlucky, but almost every time I use one of these machines with more than 3 products, the machine will get confused about the weight, or decide to randomly check me, or find another reason to lock up and have me wait for a clerk to show up and unlock it, which takes anything between 1 to 5 minutes. And I'm not the only unlucky person, either - I see it happening often enough to others, which usually reassures me that I made a good call standing in the queue for the old-fashion human-operated register.