The only people that can't kill an AT-2 are fools and poor souls that can't pen the cupola. You don't need to snipe the bloody thing when you can flank it. Hell, I've gotten an AT-2 killed in my Pz. 1 C by blocking it against rubble so it couldn't turn. Anyone with the least bit of sense isn't going to sit and fire at that frontal armor, which is why later Brit TDs are better: they can do more than sit and absorb all the shots. The AT-2 has a choice between a shitty 110-75 popgun or a Brit howitzer...
Why do people hate that popgun?
It's almost functionally identical to the T-34's 57mm, ten damage and two penetration less, sure, but a terrifyingly higher rate of fire
Is it the gun arc? Sluggishness? Poor ammo capacity? What?
Probably because, unlike the T-34, the AT-2 has to shoot at whatever the enemy lets him shoot, you don't have the mobility to really utilize it as well as the the T-34 can.
That's why you have teammates.
Heck as a AT-2 you basically serve as a battering ram in a T5 match and a tough nut to crack in a T6 match. Despite what people say about the top hat it's a pretty resiliant weakspot that requires spot on accuracy or being literally in knifefight range in a tall enough tank to penetrate.
It's balanced the same way KV-2.
Sire it has ludicrous armor/firepower but there are also weaknesses as well.
KV 2 requires teammates or proper positioning since the realod in his doomcannon is SPG grade.
AT-2 on the other hand is a giant block of armor slowly rolling towards whatever enemy it wants while gathering fire from the less clever folk indoctrinated by german tanks into thinking that lower glacis = weakspot.
They fail to penetrate so they try other popular weakspots like driver viweports so they shoot the strange circular thing right in the middle of the upper hull... but that's another "fake" as the armor there is a monolitic 200+ that eats KV-1s shots like nothing and THEN they finally fire at the top hat and either hit spot on and do damage OR miss my several milimeters and get BOUNCED (and it can bounce KV-1s shells if they fail to connect properly and promptly give up or run.
All that while being seen by my team or me and pestered by their fire OR my popgun.
Also people rarely consider the mantlet to be a weakspot so... yeah.
Also note that it usually sees T5 matches so people rarely have a clue in them.