Stingers don't take out helicopters in one hit. You need a javelin to do that. Which means you need teamwork... either multiple people with stinger missiles targeting a helicopter or have a SOFLAM up, which basically almost never happens. It's a bit frustrating as an engineer but I don't think it's unbalanced as such. After the helicopter pilot uses his counter measures, he will spend time flying low or flying away and not attacking you so it's still effective even if it doesn't result in a kill.