Have used MTD several times and found it very usable. Not quite as convenient as Carnival but still……We would go to the podium about 6:00 the first night and always get in within 10 minutes. We would be asked if we wanted to eat about the same time every night and have the same table.
As for bad service, the tips cover all the service people, not just waiters. It would be more productive to address any perceived poor service at the time of occurrence and forget about the few cents that person would get in tips from that interaction. Just leave the tips in place and if you feel you get poor service more than a couple of times (rare in my experience) look inward at your expectations.