There must have been some serious character issues that didn't come out but NFL got wind of
It 100% wasn't character.
Look how many chances Aldon Smith got; Frank Clark is still in the league, so is Joe Mixon, Kareem Hunt & host of other guys who had way worse off the field issues than GW3.
He was an undrafted FA for the Ravens the year he declared, he barely made it through training camp, but wound up getting cut. Then got picked up by the Phins & got a cut. Then got signed by the Packers in OTA's & got cut after the 2nd week of training camp.
He was overrated. He benefitted from Manny's Defense that allowed for DL's to press up the field & get pressures on the interior pocket frequently which lead to a high amount of TFL's, but also resulted in a ton of miscontained run plays that would break into the second level. It was a boom or bust system that once OC's figured out that all you had to do was Zone block & create cutback lanes you could gallop all over that Defense, especially considering the middle of the field always being left unaccounted for because Manny would send his LB's on Double A-gap Blitzes on nearly 2 out of every 3 downs a drive.
But GW3 just wasn't that good to sustain a career in the NFL. He was a very limited 1-Tech that didn't really play with good leverage & only really had one move. The reports from the Ravens camp when they signed him as an UDFA from day 1, was that he really struggled with disengaging off blocks & would often times just give up on the play once the Guard put hands on'em. It don't matter what your stats were in college, you do that in the league & you're not gonna last long.
A very large % of what DT's have to do in order to be successful is give effort. That's what is meant when you hear scouts/draft analysts etc.. talk about "motor" when they're breaking down a DLineman. Simply how much effort do you give on a play in & play out basis; and if you're not giving effort, what is the reasoning behind why. Could be conditioning, could be health, could be lack of traits, could be technique, etc..
As it pertains to Willis, there's no mystery or secret crime he committed that kept him off NFL rosters, he just wasn’t that good. Sometimes guys play their best ball in College & can't play a lick in the league & sometimes play like sh*t in college & their best ball in the league, it happens.