Very fast, very strong, but completely map-dependent in terms of their viability, Piperunners are unique, if situational units. Unit strategy will assume that a Piperunner being used will actually have pipes to travel along.
Piperunners are incredibly versatile Indirect Combat units, being able to attack any other unit in the game with the exception of dived Subs and hidden Stealths. Their combat power is not to be underestimated either, dealing high damage to any enemy unit hit, comparable in firepower to a Battleship.
The largest shortcoming of the Piperunner is its incredibly limited movement, being able to only traverse Pipes, Pipe Seams, and Bases. If a Piperunner is built on a base without any connecting pipes, it will be completely immobile, so it is not recommended to build one, even as a stationary turret. However, Piperunners are incredibly nimble, being able to travel very quickly along pipes and are not slowed by any weather conditions.
Another advantage of Piperunners is that they are somewhat easier to defend than other Indirect Combat units, thanks to any adjacent Pipe tiles being completely impassible to any opposing non-Piperunner units.
Nonetheless, they must be protected from attack as their weak armor, combined with the fact that Pipes provide no terrain stars, means they will take significant damage from most units.
Ultimately, due to the limited number of maps with viable Pipe formations, Piperunners are generally the most situational unit in the game.
The same tactics for combating other Indirect units will apply to Piperunners. Despite their incredibly high movement, they still cannot move and attack in the same turn.
Their movement is also much more predictable compared to other Indirect units as a result of their limited terrain capabilities. If possible, generally keeping away from the Pipes they can traverse will limit their effectiveness greatly, but if not, just staying out of their range will suffice.
If there is a Pipe Seam on the Piperunner's path, destroying it will also greatly hinder its mobility, as Piperunners cannot travel over broken Seams.
Tanks and B-Copters are good candidates for attacking Piperunners, being significantly cheaper, able to move farther than a Piperunner can shoot, and still capable of dealing considerable damage to them. They have a lower minimum range than Rockets, so it may be more difficult to find an opportunity to get within range. However, due to their high cost, building a Piperunner can leave your opponent deficient in more mobile or versatile units and taking advantage of this can serve as a good counter as well.