Most of the people say now that Fate 2006 is a bad adaptation. But for 2006 Standard, it was not I think, It was great.
First the visual, the animation was not that bad for that time, it was decent (the animation had really dropped when animation by computer came and until 2007/2008 for TV serie very few series had great animation, great art sometime, but animation it was rare and Fate 2006 had some great animation fight like Kuzuki vs Shirou, Rider vs Caster, Saber vs Kojiro and Saber vs Gilgamesh) and the serie was a precursor in the utilisation of lighting effect.
If you read review of the serie from 2007/8 on my animelist by exemple, You can see that most of the review praise the graphic
http://web.archive.org/web/200809080...ate/stay_night
and actually with the Wayback machine, you can view that Fate 2006 was seen as one of the most popular serie of that time,
(it stay in top 100 during 2 years and in top 50 during a full year) the rating of the serie back then was high (8,20 and it stay above 8 until Zero finished is airing) and stay that high until the release of the
and Deen series were among the most popular back then (FSN, Fruit basket, Higurashi, Kenshin tsuioku-hen and Read or Die were big hit and Deen also worked on HxH 1999)
http://web.archive.org/web/20070127065846/http://myanimelist.net/topanime.php
And actually the Fate 2006 animation director was the main Ufotable animation director of these time(Ufotable and Deen share many staff especially their early staff, Demon slayers director and chara designer by exemple came from Deen)
And it was not only the visual and the animation.
The popular series from these time were really different and it really not sure that darker series like Fate Zero or heaven UBW or Heaven Fell would have been popular.
It was the golden age of the big 3 of the shonen jump and the most popular anime series similar to FSN at this time were Shakugan no Shana, Negima and My-Hime (the comparison with these title was made back then and FSN was seen as superior), so the adaptation was made in the same spirit, more comedy, lovcom and shonen.
And don't forget that at this time Sakura character was the less popular heroin and Heaven Feel the less favorite route
For the fact it mix the routes, don't forget that it was pretty commun at this time for VN, Shuffle and the Key adaptation made the same thing.
The serie were made for newcomer and needed to give them a complete experience even if they stop at the end of the serie.
Killing 3 of the servant and Kuzuki offscreen would have been terrible for an anime only viewer, and without 2 two added episodes with Rin and Shirou fighting, Rin would have been a bland character too.
Making 3 full adaptation was not possible at this time, FSN was not that popular, it was just an Eroge (Realta nua was not released yet) and only another eroge already had a 24 episode adaptation (Shuffle).
and actually, 3 full adaptation would have been boring due to the big similarity between the begining of Fate and UBW. Even Ufotable have nearly skipped the prologue in Heaven Feel movies.
Nasu Kinoko was ok about mixing the route and Fate 2006 have very great writers:
Fumihiko Shimo and Jukki Hanada are the writers of Kyoani series who have great writing. Fumihiko Shimo have also already mixed storyline of VN before in Air adaptation (and after in Kanon and Clannad)
another of the 4 writer of the serie is Mari Okada who is one of the most well know writer in anime community.
I doubt that talentuous writers would fail to create what they wanted to tell.
The anime was not made to give a full experience of the VN but a more complete experience of the fate Route working as a stand alone (like Tsukihime manga adaptation), to make it an enjoyable serie even when you don't read the VN.
The main reason fans now dislike Fate 2006 is mainly because of the period.
As I said, the rating of the serie dropped after the release of Zero, the main problem was I was not of Fate Zero tone (where UBW try to stay) and people stating with Zero or UBW feel it's not the same universe, it's not right. especially with the graphic being old now.(and even so, now FSN is still 7,19 on myanimelist while UBW is 8,13, the difference is not that high)
For me it's pointless to says Fate 2006 is a worse adaptation than UBW, it's like saying that Burton Batman is worse than Nolan or Christopher Reeve Superman is worse than Henry Cavill or Starwars Prelogy is worse than original Prelogy.
They are just product of their time made following the standard of their time.
TypeMoon Fanbase back then was also less serious, back then, Meltyblood was the main topic for Tsukihime and it was mostly parody, Hollow Ataraxia was the last game released, the spin off was Take Moon and doujin manga by Crazy Colver Club and Sword Dancer were the hype, Type Moon fanbase was still nearly a Doujin fanbase. Even after the release of Zero novel, during the 2000, the focus on fate was still mainly in funny titles, the first true spinoff game was Tiger Collosseum, Prisma Ilya started as a parody of Sakura and Nanoha with no serious element. Even unlimited code had mini game like that
https://www.youtube.com/watch?v=xKF8hKOHzPc