The throughput rate of the kiosk is lower than that of a cashier and POS machine - it is hunt and peck for what you need - confusing flip menus and there is no quick keys like the POS for selecting condiments or addons for a meal so it takes way longer.
I rarely eat an McDonald's, but had gone inside one a few month ago that had those kiosks. I found the experience to be horrendous. Using them was slow, confusing, and extremely frustrating. I decided to eat elsewhere.
It is annnoying.