Attritioners are kinda like where warheads were. They are really, really good when there are lots of targets around, but lackluster against a few.
They are also weird in that the cap of MK IIIs had better damage per sec than any MK cap of the others (1500 MK I, 2400 MK II, 2700 MK III, 2400 MK IV, 1500 MK V), so asteroid rarity didn't help them. The only good thing was stacking them all on a planet to get an extra 10,500 DPS (per target plus feedback) on your chokepoint planet. Noticeable against Exo waves, not against anything else (anything else would die just as fast on a defended planet).
On the other hand, the Attritioner AI is loving the new hp values.