All it takes is a loss factor of a few percent, and the entire model breaks down. This is the same reason that long-range RFID (UHF RFID) never worked for cashierless checkout too. You couldn't ensure 100% coverage. Some tags just couldn't be read -- either due to adversarial tactics (eg. shielding with your body) or material properties (eg. metal) -- and the engineering countermeasures to overcome these challenges m…
I have seen supermarket chains do away with automated checkout (Jewel which parent company is Albertsons) systems because they increased loss factors.
Scanning while you shop, however, doesn't really seem to work. Tesco tried that and basically stopped it again. You either need to check really often (high staff costs) or will have losses.