This is correct, the rest is pretty heavily opinionated.
Volks were supposed to be downgraded when upgraded with shreks, that is they were supposed to suffer vs. infantry. However, when they were at close ranges the shrek reliably killed infantry models, making it an upgrade in some ways. The shrek was also extremely potent, initially locked behind a T2 unit that required huge opportunity and munitions costs to unlock. Moved onto a cheap spammable unit it created blobs that could effectively take on most threats and intimidated armor in a way that was unfair.
However, when removing shreks from volks it became obvious that OKW was short of reliable early AT. This job could only be adopted by Sturms in the current game because no other tier structure has an infantry unit that arrives at the time of light vehicle play. This is problematic as Sturms are already highly burdened with combat and engineer duties, adding AT only makes it obvious how much they do elsewhere.
As it stands, removal of shreks from volks was useful in reducing blobs of overly lethal spammable infantry for OKW. It was not part of the core design and was poorly implemented within the rest of the faction, suggesting that a more complete treatment of the issue is still needed. However, the game is not broken by the change and so we push on.
I don't know what you mean by range problems.
the other large problem with having shrecks on volks is that it allowed them to gain vet VERY quickly, making them much more resilient and better at fighting infantry (since their vet bonuses are geared towards their role as a low-cost mainline infantry unit). this, combined with 5 man squads, made them very hard to counter, especially in blobs. allied armor can't reliably deal with 4 squads of vetted volks with shrecks; neither can allied infantry kill them fast enough to get wipes.
hence the rocket arty meta.