either: "not working properly, glitchy, etc." or you can take it to mean "batshit OP, so poorly balanced it's absurd, etc."
It's this. By a long shot.
The USF minigun tanks glitch
This was so incredibly broken that it made the game basically unplayable in all modes until it was fixed. Unless you could somehow win before any USF player could rush a tank, you would lose. Not only that, it wasn't just the Sherman that could do it - the bug was (iirc) a buff stacking when crews re-entered their vehicle - so it worked with Shermans, Scotts, Stuarts, M18s, and I think even Calliopes as they still had crews. The Sherman was just the obvious choice as it was relatively cheap, had high HP, and both decent AT and AI stats - so when it switched to full auto, it tended to delete everything.
Other notable "OP/Broken" things:
1. UKF at launch. As others have said, everything in their kit was incredibly strong. Additionally, structures could brace indefinitely (the duration was longer than the cool-down), and they could be repaired at full speed while braced. As a result, picking the "Advanced Emplacement" doc meant having basically indestructible emplacements. As OST, you were effectively required to have a Mortar Halftrack in one of your docs, exclusively for the incendiary barrage. Team games were really, really bad at this time.
2. Soviet "Hit The Dirt" at (or around?) launch made units effectively immortal.
3. Soviet "Sniper Clown Car". At launch and for a while after, you could put a sniper in the scout car. The sniper was immune to counter-snipers while garrisoned, but could fire at full accuracy while the scout car was moving. This basically removed any downsides of having a sniper, while giving it incredible mobility. Your only hope (as OST) was a lucky mine, or rushing a 222/Pak.
4. OST "Laser Guided" mortar. At one point, the OST mortar was much more accurate than all other mortars in the game. This could be boosted further with various bulletins, making it effectively 'laser guided'. Any sort of support unit play was effectively impossible, and counter mortars didn't work as OST had counter-barrage on theirs.
5. OST "Jaeger Armor" Elefant stacking buffs. At one point, you could give the Elefant spotting scopes, which buffed it's LOS to something like 85. In team games, you could park an OKW Command Panther and OST Command P4 beside it to buff it even more. I believe at its worst, the Ele had something like 100 LOS, and massively increased armor.