So the topic of this thread, to me, is not really about hard rules vs guidelines. It's more a matter of transparence.
That begs the question: Transparency to what? If you mean the degree of transparency to the "rules", then we are talking about Hard Rules vs. Guidelines.
The trouble is, in my opinion, is expectations. Every player, including he DM, has different expectations. Even if everyone laid them bare, those expectations change, are not articulated well, and are interpreted poorly.
To circle this back to the OP, HP are a gaming device to determine when a combatant is no longer "in the fight". There is a base assumptions here that I don't think is valid: A DM doesn't arbitrarily assign HP but rather perfectly pre-determines HP to create encounter to create a perfect challenge. Obviously this isn't true (beyond the semantics of "perfect"). DMs typically us MM HP and just go, or if they do roll HP or pre-determine them, it's rarely with an eye towards adjusting encounter difficulty. Perhaps a BBEG gets max HP or a bunch of mooks get half, for speed of play or something. But here is the thing: By doing so, the DM is "adjusting" encounter to create a feel/challenge/narrative. Doing it on the fly or not doesn't change the fact that its up to the DM.
DMs do a LOT of things not seen by the players. They must. Transparency is good for a game of Lawyers and Litigation, but D&D doesn't need it.
What it needs is a DM you can trust. Games thus fall into these camps:
Lawyer and Litigation (Maximum transparency and RAW adherence)
Adversary and Advocating (Some Transparency leading to mistrust and/or auditing of Rules)
Loremaster and Listening (Minimum Transparency with trust and shared wonder)
(excuse the poor the alliteration) - we could sub-divide and categorize further, but the point being between the two extremes is the muddy waters (Adversary and Advocates) that at worst is mistrust and poor feelings and at best Rules auditing and questioning.
My sense is that most games fall in that middle category with either the good or bad outcome. Its hard to get 5 people on the same page, so as a rule, the muddied middle is where games are.
My advise to that muddy middle: DMs try to establish trust that your not fudging to thwart/help players but to propel the narrative if you must fudge. Players try not to get too hung up on every point of damage or bonus. DMs can always up the ante or dial it down (next encounter or on the fly) so play it straight. It is what is. For both, the muddied middle means compromise is required to keep the game going, you'll just have to accept some things that bother you or you'll have to find another game.
For those on the extremes, enjoy!