I tried exactly as you say


wptr_007 against juggle and even with 6 of them it was utterly pathetic. Juggle costs just 1700m.
Did some testing, it takes 17s to kill the juggle that costs 1700m vs 4560m for the 6 nimbus.
With equivelent cost, so 2 nimbus... it took 55s to kill the juggle. That is not a counter, that is a minor annoyance.
The juggle just walks away giving 0 shits about the nimbus.
Some comparisons:
5 rev took 10s for slightly lower cost than the 6 nimbus and higher hp
Equivalent cost of reavers (which have a similar weapon look), i.e. 22 reavers, took 4s, although obviously not a great comparison, however the reaver has 280dps and very accurate aim. Taking the same "bullets" from the reaver with large spread would make a much more reasonable nimbus
You say those units take all of their "damage" but the MAX damage output is only 180dps. If it was to make sense that it was good vs groups of units or larger units / buildings. Then the dps should be much much higher and a slightly larger spread.
All it really does is makes the enemy team spam loads of AA, which then screws over the air player.