It's complex, but basically, I find the most predictive factor in riot unit power not to be the efficiency with which it kills raiders once it's found them, but the reliability of the unit in achieving the riot vs. raider scenario.
Hunters are as fast as a leveler without the silly maneuvering.They can be where you need them to be.
Hunters have 390 sonar. They can find the otherwise stealthed ducks/snakes and wreck them.
Hunters have 1850 HP. While that's not amazing for cost, you're unlikely to lose the lynchpin riot in your force without first engaging. They're great for attrition stacking similar to reaper.
They've got good range for a riot, and great aoe with decent rate of fire, meaning that it's very difficult to micro around them.
While they don't really have much of a secondary role like other riots, they do excel at hastening the end of the raider game. Their damage is their only real limitation, and while that's important, you don't need that much damage vs. raiders.
Also, while it's true that ducks heal, they don't heal that fast. You're still tempoing any duck you damage, and raiders lose value as the gamestate progresses.