Suhiir is right, ATGMs are a serious threat to helos only if said helo has already spent some time hovering in one place (practically, more than one turn in the same hex). In that time frame, immobile helos can also be effectively targeted by sufficiently advanced tank guns. IIRC even aircraft strafing and cluster ammo can take them out, so...
Regarding MANPADS, I don't know which Russian weapons you came across, though I'd bet for basic SA-7 or similar (statistics talking). Remember that compared to modern weapons, such missiles are utter crap and not able to hit the broad side of a burn if it isn't already on fire. Against a helo with some amount of EW rating (i.e. any modern gunship), they are just a bloody nuisance, with the additional risk or dealing minor damage to your helo and sending it off-map.
Also bear in mind that in most cases IA-controlled SAMs are
meant to engage air targets as often as possible. So dedicated AA weapons + lousy accuracy gives you blind pot-shots as soon as something barely airborne pops up on the horizon. As of now, the IA isn't very good at holding fire, hence its tendency to waste precious ammo against unreachable targets. Which ATGM teams wont do that often because they won't open fire against helos at a whim. So they keep their shots for instances where the helos are much more within their target frame (i.e. stationary or nearly so), and so they get better average hit ratings. QED
(I hope)
Just for reference, what kind of ATGMs were fired at your helos with such accuracy?