Aside from micro and macro issues, i.e. losing squads by not paying attention, not setting up MG quick enough, floating 400mp and truck being out of position to set up, I think the ISG was a bad choice. Its not going to do much against 3 maxims already out on the field, at least not in the time you want it to. As a later investment, yes its worth buying.
If a sov player is maxim spamming, its safer to get a flak HT out and drive the maxims off that way. Its also a hard counter to the shocks. Against con spam it would be a risky move since AT nades will take them out in no time. Yes a zis will eventually come but not before you drive off 1 or 2 maxims, after that point, your infantry would be safe to move up and see any incoming Zis's and micro your flak HT around that. Against heavy T2, Stuka is also amazing, but I wouldn't get that as a first build, after if you get some AT in the form of shrecks/puma/raketen it would be a good choice. After that I would strive for Obers for AI and end game AT would either by a jagdp4 or panther.
I've not played OKW as much as sov or USF but I've played against them enough to know what works
