Hard (and complex) mechanics make it way easier for nitpicking ruleslawyers to argue and sometimes put the DM in difficulty over his interpretations, something that I saw way too often with 3e.
It's not inherent to hard systems, but it's a failure when a player knows the rules way better than a DM because they are complex and intricate.
I agree, but if you look at forums, some people even in here argue that they have encountered bad DMs (not in the sense that their games were poor, but that they were ill-intentioned) and that the rules were there to protect them. I must add that the relatively recent intrusion of "player agency" in the debate has made things even worse, for example look at the way
@Maxperson reacted to my descriptions of what happened to a character after the description from the DM.
I have see (way too many) forum posts about this unfortunately.
I agree, but the problems occur when there is a difference between expectations. For example a player expecting an extremely fair game and playing in a game with a "Dark Souls" DMs (I'm playing Elden Ring these days, and I think it's a bit easy mode compared to Dark Souls or Sekiro

).
While I agree in principle, this highlights the need for a proper session 0, with everyone of good faith. But with an inexperienced DM, it makes me even more furious when experienced players hit on him for not respecting their sacrosanct "player agency", while at the same time not respecting the work done by the DM in preparing the game and helping him. And this is from players on this forum, showing a level of entitlement when they become players that I find way more insufferable than what any DM has ever done to me in 42+ years of playing the game.
In this, I agree with you, it's just not a perfect world. I suspect that there is a middle ground which is the best for inexperienced DMs, with not too many clear rules that they don't get confused or bullied by ruleslawyers, but enough rules that the players don't feel that he is making too many things up, not necessarily in an equitable manner.
5e certainly is not that, the ruleset is not that light, but it is purposefully fuzzy for an open game. 4e was better in terms of having a very clear ruleset, but it was also formal and way more complex (some of the complexity is compensated by the clarity, but still)...
On this I agree, 5e is not the best. That being said, the problems above only occur for me when there are players with strong personalities. If you are just playing along with friends with no major expectations of "playing by RAW", "respecting player agency", etc. it's much easier for the DM to run a convivial game even if he is a beginner. If the players are beginners as well, they will have fewer expectations and they will feel less entitled.
See above, when I'm a player, I'm always extremely forgiving and helpful towards my DM, even if he is inexperienced, makes mistakes, etc. The DM is always right is something which is very strong for me. Even if sometimes I feel railroaded or my character a bit abused, I don't mind, I've never met a DM who did that on purpose to annoy me or asser anything, it was always honest mistakes or more simply a DM not really realising what he was doing or not knowing how to behave so that the game completely slips out of his hands (or just feeling that way).
What I really can't abide is experienced players criticising that kind of DMs, coming to the table with high expectations, a high level of entitlement and not the slightest hint of compassion or helpfulness in their heart for the DM running a game FOR THEM.