I'm not sure if it's most effective against heavies like grizzly or reapers, which are bursty and may still be able to fire ocasionally. I think it also makes them lose the disarm "damage" faster. It seems more effective against medium-high weight units with relatively lower HP/cost than the heavies, namely some ships, morphed commanders, and riot units.
some of the counters to shield balls, like thunderbirds and wyverns, can be disabled by a single or a pair of racketeer shots, which may or may not randomly happen in time when they're targeted near the front line, but will probably happen on hilly maps where the wyvern needs to maneuver closer before being able to fire.
the effect of the disarm increases as the unit gets damaged, so low hp heavies will get disarmed in one shot.
a single racketeer can disable a stinger near the enemy front line after a few seconds of exposure (enough to fire twice), and costs less than it does.
On a battle between a medium army and a bigger one with a mix of medium and heavy units, if the team that's ahead adds some racketeers, key units on the weaker side are likely to get disabled leading to a decisive defeat. If it's the other way around, they may not help much. Is this desirable?