Very interesting that some legacy CF properties will be going backwards, in a sense. I do prefer a wristband system overall. Fast Lane being phone-based is a double-edged sword.
On the one hand, from a GAdv perspective, I never had a problem with the Flash Pass system. Whether one could board immediately or had to wait a little depended on the standard queue, and I found it very fair for both users and those in the regular line. Since I only utilized singular skip-the-line passes, I more so used the reservation system as a reference for wait times, and it was always surprisingly accurate (mileage can vary on that, of course).
The downsides are plentiful though, and the locker question is a big one. Great Adventure has an odd mix of smaller "Free" locker banks for Flash Pass users at no loose article rides like JDC (and previously Kingda Ka), but for rides where loose articles are permitted, you're on your own. That policy bit me at Magic Mountain; I had 3 single-use passes that would have been very helpful, but my phone shorted out on me early in the day from an internal component presumably getting jostled loose on X2 (no exterior damage). The Flash Pass employees basically told me I was out of luck, and asked if I could use another person's phone for it. With all the 2 Factor Authentication stuff these days, I could not.
It was a real bummer. I only got to do 2 other rides that day: Scream!, and a ride over to the phone store for a temporary replacement.