You can change FP+ reservations on the fly, so if you think you are going to be late or not going at all, you can cancel them, switch them to another attraction/time, etc. For instance, we had a FP+ for Mission Space, but when we arrived at the ride there was only a 5 minute wait, so I quickly change the FP+ for another attraction and rode Mission Space standby. It's a pretty flexible system. You will also have a few minute leeway (we got on two rides 5 minutes later than our window). Just don't show up later than that. I saw a family get turned away from Peter Pan because they were an hour and a half late for their FP+ slot. If you don't think you are going to make it, change it!