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.
Namely, restaurants who move to slabnus simply get rid of the waiters who would have taken the order. You're left with even fewer waiters serving food and drinks, let alone taking orders.
The coup de grace is I don't even get a discount for the degraded service.
Note: Slabnu because I'm pecking at a slab of silicon instead of a proper menu.