Thinking about it, 5 players on every play must be ineligible receivers according to the rules of football.
They're determined essentially by their alignment. You have to be either on the end of the line of scrimmage or in the backfield. 7 men have to be on the line of scrimmage for the play to be legal, so only the 2 on the end can be eligible receivers.
In this offense, they have 2 QBs, a center, and essentially 8 wide receivers. On a given play, a total of 5 out of the 8 receivers and center must be ineligible.
I really don't have a problem with it, because as a coach, you just have to teach your players how to determine the alignment each play and figure out which players are eligible to catch a pass. They have to call the formation set at some point, and at that point, only one player can go in motion, and you should know who the 6 eligible receivers are.
I could see where it would be difficult if you were to play it man to man essentially, and your players knew which players could and couldn't catch a pass.