Yeah I used to facewreck US and Soviet light tanks with my little Flakpanzer. All you need is a couple of lucky shots, and when you're firing 450 of them a minute...
I have to disagree about the Marder III being good/alright. Thing has a damn glass barrel. First hit, ANY hit, the barrel goes black and I'm a useless piece of metal on wheels. And the profile is so high that you're a great target. Much prefer the Sturmgeschutz line. Or the Hetzer.
Compare the SU-76, which is essentially the same concept: 76mm tank destroyer with an open cupola and no turret top (in fact, it's the exact same gun). The SU-76 has lower penetration but a much lower profile and faster fire rate. The Marder has better armor in the frontal plate, but the frontal plate is about the ONLY protection the Marder has.
You have to remember that the Marder III was a stopgap solution to the German shortage of tank destroyers. The StuG III was a far superior machine but much more costly and time-consuming to build. The German answer to holding back the flood of Soviet armor showing up in late 1942 and 1943 was to slap captured Soviet 76mm anti-tank guns on captured (and by this time, mostly obsolete) Czech tank chassis, which is why it's in the PzKpfw 38(t) line.
The Hetzer was a much better "Czech tank turned tank destroyer" design, with a strong gun and low profile.