Today was a high volume day (check-in the Saturday before Thanksgiving). I expected it and still got extremely frustrated when it did not work.
That said, if your FP+ day hits on a high volume day like today, I think you should expect to have problems and plan accordingly.
The #1 piece of advice I have is to schedule the hard to get ones first. Today, it worked out for me that after I could not originally get in, I was able to get in for about 5 minutes, or just long enough to get FOP, Slinky, and Mine Train. Seriously, it was just those 3 before it crashed again. But, just getting those relieved a lot of anxiety. When it came back up again about 15 minutes later, it worked fine. But, it's entirely possible that I could have lost out on the Big 3 if I had not nabbed them first.