Marders are terrible in 1v1s. They cost far too much fuel and have too slow a turning rate to be effective. Anyone with a brain can circle your marder and destroy you and they cost too much fuel in a fuel burning faction to be mass produced.
Paciat: Of course you have to have two. But an unlucky occurrence that causes you to lose just one means you lose both. And there goes 80 fuel. They are simply too inefficient for their cost. AT nades need to be buffed somehow in order to make circle strafing marders more difficult when you go T1-T3. If you go T2-T3 you have shreks of course, but shreks in a halftrack are better/cheaper AT than a marder anyway, so you don't need the marder to begin with. Marders are pretty much only useful against the Brits where you can either truck rush him or outrange most of his emplacements.