I'm guessing it's because they think that it's rather depressing from Saber's standpoint. The Grail turned out to be something different from what she wanted, so she was unable to fulfill her wish. Not only that, she has no real purpose other than to find a new 'conclusion' to her own life by viewing the life of Shirou because she sees how Shirou is similar to her, but ended up following a different path.
It's a 'slap to the face' to fans of Fate (the route) because at the very end, Saber was proud of her life, and thus accepted the concluding result that she has lived a good life as King and did not regret anything. This is different to UBW Good where she couldn't fulfill her wishes, and was unable to arrive at the same conclusion she did in Fate (the route), thus losing her 'conclusion'. Saber was reduced to someone who's purpose in life was to find a purpose in life. At least that's what from what I understand..
Of course, as a Saber fan, I don't see any problem with UBW Good 'sunny day'. It's as close to a harem ending as Shirou can get.
Now Ilya on the other hand...