It's no secret to me that the Fate/Stay Night fandom tends to be critical towards the anime adaptations. So I watched the UBW anime, and, to my mild surprise, I really enjoyed it!
Okay, but to be fair, I read the entirety of the F/SN VN beforehand, so I knew what to expect from the UBW anime. A larger focus on the fight scenes, a smaller focus on the character writing. But I actually found that I... didn't really mind that! Maybe because I wasn't ever expecting Ufotable to reproduce an adaptation that actually matches the quality of the UBW VN route.
Shirou is obviously the character that is the most lacking in the anime as compared to the VN. But as a VN reader, I felt like I could just fill in the gaps for his nuances and subtleties as I watched the anime, because ultimately his portrayal is consistent with what I knew about him and the evolving relationship between him and his ideals in the VN. However, he also isn't quite as snarky as he is during the VN, so he brings somewhat less to the table in the Shirou x Rin romance than in the VN. That being said, Shirou is quite a difficult character to adapt: he has so, so much internal monologue, and unlike someone like, say, Kara no Kyoukai's Shiki Ryougi, he's a very conventional and generic protagonist on a purely surface level. I think there are some on this sub who might insist that it would not be a Herculean effort for a sufficiently skilled anime director to properly adapt Shirou into an anime character, but, like, I genuinely think that the number of characters like Shirou, i.e. characters in a written medium that are so reliant on their internal monologue, that receive satisfactory adaptations in a purely visual medium, are super few and far between.
A lot of what I can say about Shirou in the UBW anime can also be applied to Archer, except for the fact that he loses less monologues compared to Shirou. The dynamic of Shirou and Archer in the UBW VN is compelling and really well-written not only because of them being conceptually complicated characters, but also because the evolution and resolution of their conflict of identities is like a beautiful poem. It's a poem whose beauty is unfortunately only partially retained by the anime, but seeing Unlimited Budget Works animated makes up for it somewhat... right?
Rin became more of a tsundere than the UBW VN. But that's really not an issue, because it's the damned UBW route. The route where her magus persona gets systematically and comprehensively dismantled thanks to Shirou (and Archer, kinda). Compared to Shirou and Archer, I think Rin's portrayal in the anime was nearly perfect. But, let's be honest, while one can argue that the character of Shirou in F/SN can be decently well-understood via his character development in the UBW VN route, the same is not true for Rin Tohsaka. For her, both UBW and Heaven's Feel (VN routes) are mandatory, and nearly equally as important for her character, for the obvious reasons. So if one were to only experience the UBW route, anime or no, they would be walking away with a wildly different conception of Rin as a character compared to those who fully play the VN.
There is pretty much nothing negative I can say about the art, animation, voice performances, or soundtrack of the UBW anime.
Finally, if there's one part of the UBW anime that I am eternally thankful for its existence, it's the Episode 25 epilogue where we see 2-years-later Shirou and Rin in London. Not only is this an amazing followup to the UBW VN True Ending and a healthy amount of fanservice, but also this is a beautiful way to illustrate and elaborate on the main thesis of the Shirou x Rin romantic relationship: the two of them complete each other and support each other on their respective journeys and goals, and ensure that they do not fall back onto their worst flaws (Shirou becoming Archer, and Rin becoming a cold magus, respectively).
So, in conclusion, the UBW anime is an understandably but undeniably incomplete adaptation, but a genuinely enjoyable experience and sendoff to Shirou, Rin, and Archer as a fan of the VN, specifically the UBW route. So I'm really grateful it exists! This yap ended up way longer than I originally intended.