I think one of the major reasons that Ostheer is underrepresented is the way the rules work. The Axis player always has to declare his faction first. If you declare Ostheer, the Allied player can then "counter-pick" with USF because Ostheer struggles against USF so often on so many maps. With the way factions are in the game though, there needs to be some precedent of one player declaring faction first.
As for Ostheer underperforming, it isn't a huge surprise. The early game strength of the faction has been slowly gutted over the course of CoH 2 with nerfs to the HMG42, LMGs, FHT, tech timing, PaK 40, Pgrens etc etc. The usual justification for this was that late game Tigers/Panthers are super strong. Sadly, Ost T4 isn't feasible in a close game. Ost T3 flounders against Soviet call-ins and USF in general leaving the only option for Ostheer as Tigers. Properly managed T34/85s deal with Tigers without much trouble as long as Mark Target is available. And the new Is-2 can pretty much go head to head with a Tiger and come out on top. PaK40s are still amazing but when a faction is crutching on one late game heavy tank and their AT gun... things aren't looking good.
LMG Grens are still strong but they don't have the ability to hold the line like late game Volks with their insane Vet bonuses (plus the ease at which they get there with schrecks) and 5 man squads. Nor do LMG grens have the raw killing power of Obers. The MG34 fills the same role as the HMG42 but OKW has a better arsenal with which to support it.
Not to mention the fact that the Luchs is the perfect vehicle to tide an OKW player over before getting a Panther whereas investing into a P4 could potentially mean never Ostheer never making it to a Tiger. Plus the P4 is probably easier to kill than a Luchs with the combination of small target size, crazy killing potential, in-combat stealth and raw speed.
TL
R - Ostheer is plagued with an overly expensive teching system if you go past anything but T2. Plus their base-line tanks cannot compete with Soviet call-ins or USF Major tanks. Tigers are strong but they come late (often too late against USF). Grens are too fragile and you don't have the luxury of mass amounts of schrecks everywhere to supplement your other AT. Soviet would suffer from the same problems as Ostheer regarding tech costs except they get phenomenal doctrinal choices like Shocks, 120mm mortars, Mark Target, T34/85s and 30 munition mines - oh and demo charges.
United States
1
Relic Entertainment
