We've recently finished our first playthrough, and I must say: I'm a little disappointed. Heavy spoilers for the ending of the game, so be aware of that when reading here.
I found the reveal of the true killer kind of annoying. It's a completely random person we've not seen throughout the game yet, so there was no way of us putting pieces of information together to form a theory - it's a completely unrelated person. In addition, I didn't like the motivation he had for killing the mercenary. Sure, he's a veteran of the former revolution, he despises everything that's not communist and took a problem with a mercenary having a sexual relation with a woman he spied on for months. But that whole parasocial sexual relation, the peeping, the ultimate motivation for the killing... I don't know, kinda yucks me out and didn't feel compelling at all.
Imagine if, after the tribunal, Klaasje's disappearance was related to her actually being the killer. That would have made sense. But she was likely just afraid of how he leads would make her look like the prime suspect, so she fled. Still, I probably would have preferred her to be the killer somehow. Some kind of spy/agent working for some corrupt people who want smash the union or something.
Then the whole thing with Ruby. It was an intense encounter finding her under that ruined factory and the confrontation was quite dangerous - we almost died there. But her ultimately not being related to the killing at all apart from staging the lynching was also just kinda annoying. I get that we didn't have any other leads that made sense at the time so Ruby was the only logical suspect. But her just completely vanishing after our realising that she's not the killer felt anticlimactic.
Then the encounter with the phasmid. It was cool, I guess, but ultimately, it didn't really contribute a lot to the story, I feel like. It's insinuated that the Deserter was somehow aware of its presence on his solitary stay on the island, but you don't know for sure if he actually saw it. Who knows - maybe both Harry and Kim hallucinated. There is nothing scientifical that would explain the phasmid's existence. But even putting all of that aside, it was a little disappointing to me that you don't even see the reactions of the cryptozoologists. Felt a little robbed of that.
Then there was the ending itself. After finding out who the killer is, I was 100% sure that the game wasn't finished yet. The huge climax when you are confronted by your former colleagues felt similarly intense to the tribunal itself, and I was sure that this would lead to further development in the story. But nope. It's over, just like that. I feel like something was missing there, but I can't quite put my finger on it.
So yea, my unfiltered thoughts I had on this. It's still an amazing game and I'd really like to make another playthrough. But the ending did kinda suck to me. I've read a bit of discussion on the ending online and some points felt valid but didn't change my mind too much, overall.
Thoughts?

disco elysium is mostly driven by character and theme rather than plot. you're not expected to solve the mystery, the game is more interested in which sidequests you engage with, how you treat the other characters, and how you choose to interpret the story. the choices you make don't influence the mystery but they do influence the events of the mercenary tribunal, phasmid encounter and the final scene quite a lot. the tribunal is mostly about how well you did at the detective work and your relationship with Kim, whereas the phasmid encounter is about Harry's inner journey, then the results of both of these scenes filter through to the confrontation with the other cops.
I also think the game does a pretty good job of setting these expectations, because for most of the game you just think the hardy boys did it, instead the game works pretty hard to make you curious about other things and you end up just going through the motions of the actual investigation
the deserter is probably one of the weaker characters, he does feel like a plot device, but he's there to let the story connect the events of the game and harry's own personal journey directly and literally to the history and politics of revachol, i think that's probably why they decided to make him the killer in the first place