Yeah, the app and kiosks are slow and buggy, but I've never had a custom order prepared wrong with the kiosk or app. And as slow as it is, it's still almost always faster than ordering in person because your order is usually ready when you arrive and you never have to wait in line at the register or drive thru (assuming you choose to pick up at the counter, which you should if the dining room is open and you don't want to wait).

Also, when the app or kiosk bugs out and fails to correctly process an offer, or lose an order, McDonald's cashiers and managers are in my experience typically trained to set things right. Not only will cashiers honor "app only" deals at the register when something goes wrong, managers will occasionally comp the entire order.