I've customized our McD's orders for my entire life - they mess it up maybe 1/20 times, about the same ratio I'll have to park and wait. Otherwise it's always worked for me!
I almost always (except if there is a big queue) customize something, because then it's guaranteed made at the spot, instead of the heated old stuff. And since I started using the kiosks, the orders are always right, and fresh.
A custom order used to be a hack to be sure you got a freshly-made sandwich and not one that had been sitting in a warmer for 15 minutes, but they make everything to order now. And they still fuck them up, often.
Hard to know for a fact without knowing where I live, I'm guessing :) FWIW, it's not true at McDonalds in Spain, they definitely have popular stuff sitting behind the counter for longer than the items you customize here.
Whenever I took my kids there I told them "if you don't want it the way they make it then don't order it."