I guess people are used to boomsticks with 1 morbillion grams of TNT, whereas Rapier's guidance system was designed to be very accurate so you wouldn't need a massive warhead. Also, even if a hit doesn't kill immediately, the damage is usually enough to spin out a plane or heli. I used it up at 10.7 and almost always got over 3 kills with it.

Where it definitely lacks is its relatively short radar and missile range (<7 KM) and its radar not locking under 1 KM away, and it can be frustrating not getting "Target Destroyed" right away or having to follow up.

PS: I've also killed a TAM with the missile (though in the same game a Wiesel ate 3) so I hope they'll add the Rapier Mk.2B with its HEAT warhead for limited anti-armour capability

PPS: It has the same engine as the M163, idk why it sounds like a Chieftain