I've never mystery shopped McD's, but for a variety of reasons I worked their drive-thru in 2021 for about six weeks.
They (the company, the franchise owners, etc.) make it extremely difficult to give good customer service. My least favorite thing was that sometime around midnight, the manager would have to shut down the entire computer system to reconcile the money, for a half-hour to an hour. During this time, anyone who came to the drive-thru had to be told 1) we're only taking cash right now, and 2) we need you to pull all the way up to the food delivery window to place your order. We had to write it down on paper, give the order to the kitchen on paper, painstakingly figure out the prices with tax when we couldn't look up prices on the system (and believe me, there are a zillion things on that freakin' menu), and then let the customer sit there and wait at that window the entire time the order was being prepared, while meanwhile there are 4 cars behind them waiting to do the same. One customer might be at that window for 10-15 minutes. It sucked.
It was also an issue that even when I was taking orders from 2 drive-thru lanes and taking money at the first window, I was expected to wash dishes in between customers. I would run over to the sink, turn on the water, get my hands wet, then hear the "ding" of the drive-through lane.
So yeah. I can kind of see why they don't want us mystery shopping places we've recently worked!