The trouble is that, allegedly all of Merlin's parks have set limitations on how many Fastracks are sold per attraction per day (although I'm sure they bend these to suit themselves when they want to, e.g. Fright Nights). This is supposed to be set so that the main queue still moves at a fair pace, and so that it doesn't have a bad effect on the efficient operation of the rides.
When you start adding other things such as priority passes, unlimited Fastrack packages, Reserve N' Queue Ride and now VIP passholders you have a problem, and a right to complain about it. A lot of this could be sorted out by selling less Fastracks per day and fairer batching, but it's the classic thing of putting profit before guest satisfaction.