After having played many games in 2vs2 AT recently with both Axis and Allies I think Sim City is indeed counterable in this game mode (double OKW works for sure). You will need a different mindset and strategy though to break through.
To be honest, Sim City separates those ones who simply blobbed their way to victory with Axis from these ones who are capable of using their brain.
By the way: Playing versus Sim City was the only challenge that I have faced in 2vs2 AT as double OKW.
It might be frustrating to fight against but seems to be the only reliable counter to halt double OKW's might in late game wherefore I am not enraged about anyone building a city against us.
Key points here are AT. If you are playing with a random, half the game is trying to get your ally to work with you to take down sim city. If you have a buddy that you play with and communicate with in the game then the effects of sim city are largely negated. Although if both sides are AT, then sim city is stil very hard to deal with.
Summary:
Sim city is super strong if...
Allies are AT and axis are randoms
Allies and axis are randoms
Allies and axis are AT
Sim city is easy to counter if...
Allies are randoms and Axis are AT
Sim city requires cooperation to take down as axis and lack of it from the allies.