What almost all people forget when talking about buff gunners, they forget the amoured glas windows they had.
But an amoured glas window has one big disadvantage. One hit will make it almost impossible to look through.
I once took a look into an LW amoured glas test. There they tested amoured glas from almost any allied plane.
They shot 7,9 mm rounds at them.
And the resulting damage almost always lead to an incredible reduction in visibility.
Now imagine you are in the tail turret and just one damned round strikes your amoured glas, after than you are practicly blind for any gunning purpose.
Also gunners seem to have been very vulnerable to fire, Willy Reschke who flew the FW190 Sturmbock always attacked from dead 6, his first shot with MGs only, would always go for the tail gunner, than he would procede to pepper the inboard engines and fuel tanks with heavy cannon rounds.