My apologies for not answering you earlier; it's been a hell of a week!
I disagree that a hive's...hive-iness is dependent on some sort of prerequisite level of misery.
On the one hand, I would argue that one doesn't need to qualify that the conditions that define a hive city are inimical to a decent life for any but the top strata of society.
But anyways, Games Workshop has done just that. From the latest Core Rulebook:
The massive populations of hive worlds periodically become unmanageable, as the masses boil over against their constant repression. Such bustling mega-cities are always rife with anarchic and destructive forces that ensure only the hardiest can survive. Yet this too works for the Imperium, for their tithes supply rich sources of fighting men for the Imperial Guard.
The only other reference in the last two rulebooks to make a specific statement about the hives (beyond just their size and scale) is similarly negative:
Examples of recovered STC template technology still being built and in use today include
... the Atmospheric Pumps that still keep the air (almost) breathable in even the largest hive-blocks.
Going back to the fifth edition? It's similarly dreary:
Centuries of industrial production have left a hive world's surface inhospitable, with toxic fumes and parched soil. Each hive world is home to many hundred billion citizens, crammed into towering urban conglomerations, known as hives, ...
Where your specific examples are concerned?
Helsreach was a reasonable place to live, so it seemed, on the whole.
I'm not sure why you think that a hive named after a mythological hell in a world that's practically unlivable, where existence is defined by quotas for war materiel production or serving as a soldier, would be a "reasonable place to live, on the whole."
Beyond those qualifiers, which can be found in a plethora of Games Workshop products, you have some helpful contributions from the novel Helsreach:
“The air still tasted like a latrine, though. And it didn’t exactly smell any better. The joys of high sulphur content in the atmosphere.”
Excerpt From: Aaron Dembski-Bowden. “Helsreach.” iBooks.
And beyond that? Well, neither Necropolis
nor Sabbat Martyr
focus on the quality of life within Vervunhive or Beati City. If people don't seem ruthlessly oppressed or miserable, well, I'd chalk it up to the fact that their immediate attention is on waging a war against horrific enemies who want to see their bodies and souls corrupted and/or destroyed.
Bottom line, I don't doubt that there is some exception to the rule out there. There may very well be a hive city with tolerant rulers, great living conditions (albeit in a giant metropolis), etc. Within the context of 40k, of course, that exception would be like Alpha Shalish: an enlightened place doomed to a horrific fate. Either way, though, it's not the standard. The standard by which hive worlds and the hives that house their population is, unfortunately, a miserable one.