Some of the show's weak points were unavoidable - the original material's storyline quality is dubious at times and the fact that Unlimited Blade Works as a second route of Visual Novel is supposed to be built up over experiences from the first one definitely doesn't help when someone is adapting it. But VN's weaknesses aside, its premise is interesting enough to make an entertaining show with lots of cool fights. And first season of UBW managed to do that fairly well.
Second season, on the other hand, does almost everything wrong. The animation is still gorgeous, but all the rest, oh girl. First of all - the creators seem to think that main strength of F/SN UBW is its story and, inevitably, character of Shirou. It's a massive problem, because Shirou is really not that great (which essentially means: he sucks). He wasn't that great in original VN, neither in F/SN 2006 and he still isn't in UBW. But now not because of his sexism or blandness but because his central conflict (which is the central conflict of the story now) is shamelessly moronic. What's much, much worse (moronic conflicts in action flicks aren't necessarily a bad thing) - the series abandons much of the action and characters' interactions in favor of explaining his utterly idiotic conflict to the viewer. Explaining again and again by two, three, four (I've lost count) episodes of pure talking. And by the end of this talking they still explained not very much - the viewer gets crazy amount of plot holes and plot conveniences instead. It's very clearly inherited from visual novel, but transfer of this style of exposition and this lazy writing into TV is inexcusable. The exact same problems haunt the interactions with and exposition of the main villain of the anime. Both these arcs are just plain bad - and because they happen to be the final ones they basically kill the show.
Also, as a result of putting emphasis on Shirou and his internal struggles, the show wastes other, much more interesting characters. Rin and especially Saber are sent off to backgrounds and play a role of damsels in distress. Rin in second part of the season continuously acts out of her supposed character. Saber on the other hand is literally doing nothing - which was explainable in the VN (she had her own route) but in the stand-alone anime it looks bizarre, to say the least. The anime even manages to botch the highlight part of its fictional universe - the moment Saber screams "EXCALIBUR!!!" isn't particularly memorable in this version. Which is really a shame.
The other part which seems to be forgotten by writers is the romance part of the original material and constitutes another example of character wasting - it started off quite well but it gets abandoned in favor of Shirou's struggles for many episodes. And then the series returns to it for a brief moment, in most convoluted sex-scene replacement I've had an honor to witness. This replacement is out of place, out of character and completely out of story's coherence. And honestly, if the series wasn't bad in the first place, its low score would be warranted on virtue of this scene alone - the very idea that, gasp, physical contact between characters (even in form of a kiss) is something which needs to be replaced in the show for teens is insulting and actively helps to make the world a worse place. The only thing which could be said in defense of the replacement is the fact that the actual sex-scene from VN was also insulting, out of character and degrading to women. But certainly it doesn't justify horridness of the replacement. Shame on you, ufotable.
All of the above maybe wouldn't be that annoying if only the action part of the second season was any good. It isn't. Most fights are static, anti-climactic and/or resolved too fast and constantly interrupted by excessive talking. I must also note uninspired choreography and lackluster design of some of the characters outfits. What's really weird - many earlier ones are actually not that bad, but in this anime the more important given fight is the more underwhelming it will be. Very peculiar and quite infuriating. The Gilgamesh's way of fighting is partially to blame for that, but still, zero imagination on the creators' side certainly doesn't help.
In summary: the series obscures the strengths of original novel (entertaining premise, cool servants, fascinating fights, good side-characters, romance), instead it emphasizes its weaknesses (main story, Shirou), amplifying it by ridiculous amounts of "tell, don't show". And as a bonus it damages the society by promoting petite bourgeois morality.
Finally, this show is nothing more than a proof that world doesn't need any more Fate/Stay Night adaptations.
