I'm an admittedly biased reviewer of this story considering I've read the original YU-NO visual novel from 1996. However, I don't believe this disqualifies me from giving an accurate review of the anime.
YU-NO is unique in that as the 'grandfather of the modern visual novel', its story is much of a prototype of some of the most popular time-travel/sci-fi stories today. It has influenced numerous highly regarded works such as Steins;Gate, Island, and even Clannad. This review, however, is about how I personally feel about the story of YU-NO being told in this anime adaption and why.
This story starts out as a slow burner, which does pay off later when the development somewhat is on point. The positive parts of the story are the many shots of foreshadowing - dropping hints on things that could become very important pieces of the puzzle as we progress further into the story. Through the first arc, we see hints and glimpses towards what the protagonist is getting himself into and what he'll be trying to achieve. We are given parts and fragments throughout the beginning few arcs that seemingly are unrelated but ultimately tie together later on. The first arc I feel was a great way of introducing the characters, the setting, and a general idea of the mysteries. Through discovering the suffering and pain Takuya's stemother Ayumi has been through, Takuya is able to not only help her resist the urge to succumb from the pain, but grow closer to her and by doing so, stumble upon information regarding GeoTechnics, their research, his father's life, what his father's goal may have been, and above all, how and why his father has given him a device that he can use to travel in the past to different worldlines.
The story itself isn't one for the faint-hearted. It's dark, brutally realistic, and deals with mature concepts and themes in a respectful manner that most other anime tend to completely avoid. The foreshadowing pays dividends, as many of the things that have seemed confusing in previous episodes now make sense in context with information that is later revealed.
If I had to describe the story with one word, it would be "complex". The two "halves" of the story are very isolated from one another, yet at the end combine back together into one to make a complete package.
Unfortunately the script-writing really falls flat with the atmosphere and tone. Scenes that were truly haunting and bone chilling, filling you with a sense of dread in the visual novel, have lost much of their impact and it really shows because many of the big plot reveals are "so what, why should we care?" moments, and it sucks because the story is so much better than how it was adapted. Furthermore, some of the changes to the plot resulted in some very glaring plot-holes that didn't exist in the original story, and were never actually addressed leaving you wondering what the script-writers were thinking.
What frustrated me the most is how senselessly some of the scenes were adapted. It's almost as if they completely changed the tone, trying to add comedy when really they overdid it and it came off as stupid and tone-deaf. The comedy when done well was fine, they just inserted it too much into some of the more serious scenes, which wasn't a problem in the visual novel because it was more "dry humor" than "idiotic humor".
Ultimately, for YU-NO to be properly adapted, it would have required most likely a) a different studio and b) more episodes in order to truly flesh out the plot and give impact to the many thrilling scenes that were in the visual novel.
In the end, the changing of the plot to fit anime format wasn't done terribly, and this is one of the better adapted visual novel stories I've seen. It simply falls flat in areas that, if had been done with much thought and expertise, could have really elevated the plot to the level that it was in the visual novel. The script-writing dropped the ball, honestly. It's a shame because a 10/10 story and plot was reduced to at best a 6/10.
If there's one pretty positive part of YU-NO, in my opinion it would be the diverse character cast. Unlike most anime with a high school protagonist and a cast of all high schoolers, YU-NO actually surrounds the protagonist with more adult characters than high school-aged characters. Because the characters surrounding Takuya are professionals with real-world life experience, this gives more credibility to scenarios where Takuya finds himself in danger or encounters obstacles a high schooler usually wouldn't.
Takuya himself is quite possibly the furthest thing from a self-insert protagonist that you could encounter in anime. His personality is defined: nonchalant, almost in a way that makes him seem like a huge jerk, as he's constantly making wisecracks to the dismay of others. It might seem as though he doesn't care about the people in his life with his flippant remarks and the way he's constantly having people do things for him (mainly Yuuki), but his carefree attitude in a way is almost an advantage for him because when he gets serious, people almost immediately notice it and take him seriously. Also, by acting like a complete buffoon, he's also able to play off some more inquisitive questions as a joke without others suspecting that he's trying to search for answers.
All of the other characters are well-built and each have their own motives for their actions. The one drawback, however, is that the characters are mainly used as plot devices to advance the story - Takuya learns valuable pieces of information about the world and mysteries surrounding him through interacting with each character during their individual arcs, and the arcs focus on the story more than developing the characters, which causes some of them to fall flat.
And because the story moves so fast, there's barely enough time to develop much attachment at all to the wide cast, leaving you to wonder why you should even care about them at all, and making them feel disposable. The sadder character moments in the visual novel actually had impact because the characters were more fleshed-out and the plot wasn't advancing at break-neck speed. Here, however, again, the sadder moments just felt gruesome and unnecessary and had little to no impact.
Ohhhh boy, here we go. The first few episodes were very dry and barren of music, negatively impacting my experience of the story. Even when the sountrack expanded later throughout the show, it was repetitive and lacking. I expected way more from Evan Call, the composer of the Violet Evergarden OST.
The OST from the visual novel is one of my most favorite OSTs ever. They used exactly THREE TRACKS from it, and instead opted for an anime-original soundtrack for the rest. That really upset me. Even without comparing the anime OST to the VN OST, the anime OST clearly is not good enough.
Music is a valuable tool for emotional impact, and the OST in my opinion didn't cut it at all whatsoever.
The voice acting is absolutely fantastic. Specifically Kaori Nazuka, as Ayumi Arima, has portrayed incredible emotion and is acting with a lot of passiont. Nothing bad to say about the rest of the cast. Maaya Uchida is great as always and Rie Kugimiya, the tsundere queen, has returned - albeit not with many appearances, but she's back nonetheless! In the second half of the series with characters that I won't mention, the voice acting was truly incredible. Very passionately done all across the board.
The sound effects...have had some wonky moments. I'll leave it at that.
OP/EDs are all fantastic. Konomi Suzuki in particular has done an incredible job with the ED1 and OP2. The second ending is probably my favorite of the four themes!
The art itself is a 7/10. Love the detailed backgrounds and general drawings. The character model changes weren't that great because they're much more generic than they were before, removing the "gritty" feeling that they once had. The animation, however, is a 3/10. This adaptation is basically a slideshow, the animation is that non-existent. The story deserved so much better.
For a fan of the original visual novel, I can say that while this adaptation dropped the ball, it was at least an enjoyable watch. I would recommend it to people who like mysteries with a lot of plot twists, but not to anyone that wants an entirely coherent story. Although it is not a true 1:1 adaptation, changing and reordering quite a bit of scenes, the way they adapted the story - showcasing all the important information - only cut out some unnecessary scenes.
This wasn't what I was hoping for out of this adaptation, but it was average! If you want the optimal experience, I do highly recommend reading either the original PC-98 port or the remake that's been released on Steam. The story will actually blow your mind.