Lack of a pintle-mounted MG. A 7.92mm MG doesn't help much against IL-2s, but Nikis and Hurris can sure be killed or disabled with one. It's also by far the best way to kill troops on the run.
use the coaxial machinegun! ya know, the one ya fire from the main gun sight with yer secondary button.. its great!
The t34 turret moves quick enough its pretty much better than a pintle mg, cuz when ya start hitting with the bb's, fire a HE from the main gun and a good % of the time -- POOF goes airplane. I have a film where I killed 8 or 10 planes in flight with a t34/76 with that coax/maingun technique..
as for killin shermans... just hope you can sneak up close indeed!