There are a number of reasons spread guys fail. Decision making, accuracy, and arm strength are the ones that are on the top of my mind right now.
Here is the list for career passing completion % leaders in NCAA history.
https://www.sports-reference.com/cfb/leaders/pass-cmp-pct-player-career.html
From that list, I didn't think Johnny Manziel was especially accurate. Brandon Weeden wasn't very accurate once he was asked to make tight window throws in the NFL. Brett Hundley is at 67.45%, and I don't think of him as an accurate passer. I think Baker Mayfield benefits from Lincoln Riley's system to get guys WIDE OPEN, more than the accuracy of his throws. I would say the same for McKenzie Milton at UCF.