The Marder's ridiculous DPM combined with PELE ammunition means that it can effectively do damage to anything, anywhere. Even the fronts of well-armored opponents can be "spalled" with PELE.
It's rather slow for an AFV, but it makes up for what it lacks in mobility with decent armor and an infantry complement.
Its armor is rather resilient against autocannon fire, but vulnerable against large-caliber APFSDS and especially against ATGMs. Not designed to be a frontline brawler for sure, but it can get out of tough situations with "double-smoke" and/or sheer DPM. An excellent support vehicle if you can effectively use friendlies (preferably MBTs ;) as cover.
TL;DR: Good in PvP, great in PVE, and ludicrous in GlOps.