Thats true. Especially against fast movers like jets. True engament time window is so small, that manpand like SA-18 has hard time even locking correctly in that time. When they have locked target usually is out of effective range. Also as I said, if they are dropping flares, then seeker has hard time to get "correct" heat singature and may lock to flare. Even after all this, if you manage to hit, warhead is relatively small so target may still fly away (as we all have seen pictures from damaged aircrafts hit by manpands).
BTW; survived yesterday 4 hits from SA-16 in game. Fifth hit brought chopper down ;) (all four where bottom and last on to left engine)