T-Mart was a mid 50's rated ATH who wasn't getting much love as a QB. Nebraska took a flier on him. I think they got the most out of his abilities. As raw a QB as you could get his RSFR year. As a JR he had a 63% comp rate, threw for 2900 yards and rushed for 1,000 yards. He threw too many picks even as a JR but he played with a gunslinger mentality that got him in trouble. Got hurt as a SR and hardly played.
Tommy Armstrong had a so-so SO year. Threw for 2700 yards, rushed for 700 but had a fairly low 53% comp rate. He was a top-10 dual threat QB but not a guy with a ton of offers or a lot of fanfare. Nebraska didn't have a group of dynamic receivers and outside of Abdullah most people couldn't name a skill guy on their offense.
Tom Herman had Braxton Miller, the #1 dual threat QB. His SO year he threw for 2000 and rushed for 1200 with a 58% comp rate. I don't think that is light years better than Armstrong's SO year. As a JR Miller threw for 2000 and rushed for 1000 with a 63% comp rate. Martinez actually put up much better passing numbers as a JR than Miller did.
JT Barrett is probably the best QB between the four we are discussing. Better passer than Miller, Martinez or Armstrong. Not as fast as Miller and Martinez but a better runner than Armstrong. No surprise he had better stats than those three either. To be fair though, he also had the best cast of skill guys to distribute the ball to as well.