the DSHK is considered a good machine gun by most people, although the firing arc is really restrictive
it could use a slightly larger firing arc
That is the problem though, the firing arc sucks considering it is doctrinal and more expensive than the maxim it should be better but it is equal if not worse to the maxim. It is not terrible but Imagine if the Command Panther from OKW didnt have the command aura and was just a regular panther with a call in that wasn't any better than the stock variant that is what the DHSK is. The marginal improve in stats that it gets is off set by the higher reinforcement cost, higher upfront cost, significantly worse ability and Vet 1 compared to Maxim.
Now that is not to say it is bad, it is decent but considering the Doctrinal/Cost should at least be close to MG-34 or even Mg-42 in terms of performance. At the moment its just Maxim for people who can't be bothered to use Suppressing Fire which is not good enough.
Even if Maxim was the worst MG in the game (its not) but lets just say that it was, it would make sense considering Soviets are the only faction with two different HMGs with the DHSK being a clear upgrade path which at the moment it isn't.
Also speaking of Upgrades the best way to make Maxim and DSHK "better" is to allow it to be upgraded to DSHK for say 60 Ammo or so and balance it around that.