It certainly comes from the fans who have played the visual novel (VN) and set the expectation too high for the anime adaptation.
Some background, the original Tsukihime VN was stated as an exceptionally big hit for a doujin game in Japan and drew big attention when an anime adaptation was decided. The VN itself also seemed to receive global recognition. Note that it also consists of multiple endings and many backstories and may take more than 30 hours (according to VNDB) to finish all the routes.
Putting aside opinions, the anime was broadcasted for 12 episodes based on a certain character's route:
"Arcueid's True End"
However, due to the limitation on episode counts and the staffs having troubles in trying to incorporate all elements from the original source, many unimportant settings were left out. Some settings and characters' personality were also changed partially. Moreover, some voice actors were not experienced in anime voice acting, including Nabatame Hitomi (Arcueid's voice actor) making Tsukihime as the debut in TV anime voice acting. Additionally, all of the voice actors were different compared to the Melty Blood which had been released before the anime.
On the other hand, the manga adaptation following the same anime's route was also released at nearly the same time and was serialized for 7 years. While the story development was interwoven with multiple routes, the manga also integrated some references from Tsukihime Tokuhon and Kagetsu Tohya without feeling out of place and losing the original source's atmosphere. Even words of acclaims from the original author were written on the obi.
Now, back to the fans' opinions, there were many reasons why they didn't want to "acknowledge the existence" of this anime. To summarize it:
- Plot development: all over the place, slow pace, wrong focus, under-/unexplained details, bizarre use of flashbacks
- Bad presentation: too many non-subtle dialogs instead of actions
- Characters: modified personality and roles
- Visual: too many close-up shots of faces/upper torsos, stiff/stilted/ motionless, unremarkable action scenes
- Audio: over-the-top sound effects, poor translation & dub
and many concluded that it's okay as a standalone work but terrible as an adaptation.
Interestingly, Japanese fans also didn't want to acknowledge the anime's existence. Most of them assumed the anime as "a dark history" and "there's no anime adaptation". Since they had the same opinion as with the global audiences, it's probably safe to say that it has spread since the original TV broadcast in 2003.