I don't like dark works in general. I usually go watch them for the sake of action and humans dying. But Oyasumi Punpun is hard for me to really interpret as dark, because...it's too realistic. I find it hilarious how realistic it is reading through it tbh, just going all "oh yeah that's real life alright!!" all the time. This isn't a slice of life, this is a half-sized main course of life with no sugar added.
The middle schoolers going about gossiping about how huge the dick of that guy in class is? Real.
The moment someone younger tries to cheer you up by giving out some newfound wisdom after a unique experience, only for them to be rejected with the response saying they don't know shit? Real.
The "I'm tired as fuck of this clingy old bitch, dat jailbait pussay tho" attitude older men have? Real.
That moment where when you just ask some mundane question like "oh, what are your hobbies" and someone tries to look all noble and shit by going on to talk about the wonder of philanthropy and the purpose it gives you? Real.
Punpun seeming to go through some incredible emotional trouble, only for it to turn out to be him really, really wanting to have sex? Real.
The crazy-ass I'm-sick-of-this-shit purposeless people out there who go about on a fit trying to kill themselves only to say "OMG I DON'T WANNA DIE" after failing to commit suicide/accidentally almost dying? Real.
The uncomfortable as hell atmospheres where sexually frustrated people will try to rape you because they feel like the mood gave them implied consent? Real.
I think the only really unrealistic/questionable thing I've seen so far that wasn't around the beginning (where weird shit happens just because) or whatever those Pegasus guys are doing (some sideplot thing) is when some guy attempted murder on the mother of the girl he's into because said girl gave him a blowjob as payment for the task. Unrealistic/questionable mostly because most people get cold feet trying, one other character was shown having it too when they were supposed to push a guy into a train's path in exchange for a lot of money, and that was during their "I can do anything, IDGAF" phase. (also real)
But normally I don't really value realism either. In most contexts where people bitch about the lack of it I really don't give a shit. But it's different in Oyasumi Punpun, mainly because 1: it says fuck you to conventional characterization (people do irrational things out of the blue, character development takes forever/doesn't even stick, they have motivations or troubles considered petty/immature, they don't follow through with a lot of what they say just because humans are cowards or fickle rather than any deeper reason, etc.) and 2: the sheer amount of realism, everywhere, makes it a lot easier to believe what I don't know about reality, like dying moms telling you that even though frankly they were shitty as hell toward you they still loved you(although like Punpun...I wouldn't care) and former Yakuza going about telling you that what the guilty need isn't punishment, but the pain of being forgiven.
It doesn't bullshit you. That's what it has going for it.