Late to the party on this, but I've held off dunking on Hermes long enough (as in, it's been 3 days at least).
In a way you could say Venat is hogging the spotlight for Endwalker complaints because the discourse around her is very polarized, and the fact that the game heavily pushes her as a hero is suuuper egregious. Hermes, meanwhile, while I do think the game justifies him too much, it seems the playerbase's opinion of him is a lot more ambivalent. I found it's at least rather common to find people dunking on his dubious scientific methods and finding the writing around him weird/unrelatable. Of course, you then have the typical Twitter creature (or OF troll, god forbid) who thinks he was a soff boi and the only human Ancient and totally relatable and you don't understand depression, but I think this is a minority opinion.
While I have myself thought of him as depressed or neurodivergent, I fully agree that this didn't seem to be the intention at all. Both YoshiP and Ishikawa are pushing him as judging humanity in a "fair" way which presumably implies rationality. The reason why I do headcanon him as depressed (as his Ancient self anyway) is so I can make sense of him, because I swear this character ping-pongs uncomfortably between extremes.
He cares a LOOOOOoooot (too much) about bad concepts like the unstable charybdis and the murder wolves, but he doesn't bother mourning the alkonosts they murdered? Meteion obviously wants to shield his fragile psyche from her sisters' report and is in extreme distress but he doesn't do anything to abort the entire thing?
He snaps (holes himself up into Ktisis), then he unsnaps (seems about ready to hand himself and Meteion in after the fight), then he snaps again (KAIROS), and then he... unsnaps again, as we are told he fought against the Final Days in earnest even though he resents humanity so much??? I'm sorry, but I don't believe wiping his memories of the last couple of days would suddenly not make him an edgy, resentful militant vegan. Am I supposed to believe his OMG-I-CANNOT-COPE-WITH-DEATH-IN-ANYWAY self would support the idea of Zodiark? The first, the second, and most egregiously of all, the infamous third sacrifice (which we have so little concrete information on, it regularly makes Venat supporters go on tangents even though it really is beside the point as far as her motivations go according to the most recent Q&A)? I suppose Hermes, of all people, going with the third sacrifice is like... the single strongest headcanon evidence for the stupid Zodiark tempering having any effect at all on a character lmao.
I can reason my Hermes-was-ill headcanon as such:
1. It serves to explain some inconsistencies in his writing when you can handily invoke cognitive bias.
2. Him eventually receiving appropriate care and treatment and eventually getting better and opening up would be a working explanation for his weird 180° turn on defending humanity, as well as justifying why they eventually accepted the guy to the Seat of Fandaniel, because holy expletive I cannot imagine this dude whom I have just watched have meltdowns every other scene in Elpis would actually seat on the highest council of mankind in any sane world. Hence I cope by imagining everyone really did notice that HERMES IS LITERALLY UNWELL.
3. It makes Ancients way more sympathetic.
Honestly I kind of resent it when people say the game unambiguously presented him as wrong. No, the gaslight was fairly strong around him too. HERMES IS SAD HUMANS DO NOT RESPECT ANIMAL LIVES! CAMERA PANS TO A LITTLE FAMILY OF ADORABLE HEDGEHOGS. Oh this is definitely the sort of subtle storytelling I came here for! His reasoning for "testing humanity" and the whole thing about it being "fair" in any sort of way (see your 3rd point lmao) worked well enough on some people who fully bought into it, at least. Probably Ishikawa herself, to begin with.
Yes and it's unironically a far more understandable motivation for him snapping than his largely unrelatable existential quandary straight out of /r/im14andthisisdeep. WHAT DO YOU MEAN I'M ACTUALLY REALLY WELL-OFF METEION??? THEN WHY DO I FEEL SO SAD???????? TRULY NOBODY UNDERSTANDS ME!!!!!!!!!!
Special mention to his dumb lines about what if humanity ever reaches perfection, will they all die with a smile on their faces then? (INSERT NEON LIGHT ARROWS POINTING TO THE PLENTY IN THE DEAD ENDS HERE) What kind of abstract question is this, you smooth-brain? I am so disappointed Emet-Selch didn't bluntly reply "Yes" like the Chad gamer meme. Would you rather be hit with the horrific Grebulof pandemic that turns you into a walking boil? Or perhaps the planetwide world war? I'll take the peaceful suicide with a smile on my face dude tyvm.
Not so much an issue as a mitigating factor that IMO absolutely should be brought up whenever Omega's lines are referenced: Omega was a person, now machine, that lived in a constantly at-war culture and a survival-of-the-fittest hellscape, whose very design is to constantly evolve. I'm not exactly surprised then it would come up with "ackshually Hermes good because brutality results in evolution".
I'm more disturbed by Ishikawa herself asking whether he was the first step for mankind or not, and I am free to reject that, just like their shallow Plenty parallel. (Ra-la take me, though.)