Fields left because he couldn't beat Fromm for the starting job and Fields' daddy got mad. Fields HS qb mentality, if the first receiver is covered, run, didn't work well in the SEC but in the Big Slow 10, Fields was athletic enough to be effective. Burrow wasn't good enough to win the starting job at Ohio State and he caught lightning in a bottle at LSU with Ensminger as OC and qb coach. He was surrounded by incredible talent at LSU on both sides of the ball. If he had transferred to, say Iowa, no one would have thought he was anything special. Ewers committed to Texas before flipping to Ohio State where he played in one game. Hurts was on 3 Bama teams, 2 as the starter and came in to lead Bama to the SECC win, hardly buried on the depth chart. Hurts graduated and transferred to OK.Transfer QBs off the top of my head that were the center of their team’s success in the past few years. Burrow, Ewers, Hurts (OU), and Fields. They all left bc they were down on the depth chart.
There have been loads of transfer qbs who made no difference in their new team's fortunes but they were able to start because they were better than the qb the team had before.