I tried to think of a great QB that succeeded in an awful system or played for a horrible team and I can't think of any. It is easier to find QBs who have left horrible teams/organizations/coaches and thrived at their next location than you can those who had to play through crap
Just from the limited games that I saw on tape, I believe the he could have been one of the best but as you pointed out, terrible teams usually drowns greatness. Most of the successful QBs usually had stable coaching, a proven scheme, a winning culture, and a chance to develop.