Starting with the theory-crafting,
Shieldbots are weak outside of shieldballs. They also lack an efficient artillery to counter mobile artillery.
That leaves rovers with terrifying badgers to counter enemy's artillery, dominatrixes to mix into the shieldball, and impalers to soften up enemy's defenses. Impalers should work incredibly well in combination with racketeers, since racketeers can invalidate a shield, while the the impalers can wipe out the shield emitter with ease. (A combination I should try)
Tanks, on the other hand, have the magnificent kodachi which can deal with raider groups. But kodachis don't synergize well with scorchers....
I guess that tanks can benefit from rovers' ability to do painful raids for cheap, whereas tanks would need to send a heavy unit deep into the enemy's base to deal significant damage. This means that a tank player with rover starter can keep up meaningful pressure late in the game, if he keeps the rover factory, using cheap ravagers, impalers, scorchers, and fencers if he dares. Dominatrixes are out of question, they're only effective in defense.
In total, the rover factory has lots of units desired by a shieldball.
Additionally, the rover can apply far cheaper raid pressure than the tank factory can.
I'll need to experiment with all of this in a match.
