Honestly, VNs as a whole are tricky as hell to turn into anime. Example: Fate/stay night. The majority of Shirou's character development in the Fate route comes from inner monologues- which would translate horrifically to anime- which is why, when DEEN adapted that route alone for the anime he came across as an unbelievable idiot. (He totally is, but the VN made it bearable)
And then the movie took the Unlimited Blade Works route and compressed it into two hours of confusion.
It does not help that the three routes are basically required to be done in the order the game unlocks them in, and Fate is the first route. UBW is the second one, and as such the movie skips a chunk of really necessary exposition Fate gave. And then Heaven's Feel (the third and last route) needs both to make any sort of sense. Even worse is that HF has arguably the best Determinator moment in the history of VNs (spoilered below).
It doesn't hurt that Heaven's Feel has this magnificence as your endgame backup.