If you want to counter a brummbar, you need a TD. AT guns won't scratch the paintwork on it. I don't think that the brummbar is OP, but it definitely plays into the USF/Sov infantry weaknesses. Both rifles and cons are primarily short to medium range, meaning that they have to come in close to the enemy infantry hugging the brummbar, hence less time to react once the fat bastard fires (you can dodge max range brummbar shots). ZiS does not have the following problem, but the USF AT gun will bounce a lot on the vet0 brummbar, even more so on vet2 (had a game on Mud where my AT bounced 6 shots in a row on brummbar, bad RNG sure but the stats don't really allow for good RNG)
correction: a single zis gun wont really cut it if you, god forbid, have no tanks + snares to scare it away
+ brummbar barrage shoots 152mm death cannons at an ~1.8 second interval, and two to three hits reliably kill an AT gun. Pre nerf zis barrage was considered overpowered at it's 2.0 second firing interval.