Heaven's Feel, especially part III, really cemented a common pattern in the Fate series for me:
Start off with an interesting, but understandable, premise
Set things in motion, maybe hint at twists
Go off the rails with things that some would consider "ass-pulls", but do it with such hyped up animation/soundtrack (nine-lives blade works comes to mind), that it doesn't matter anymore
Sort of try to wrap it all up, fail
Create another route, prequel, or alternate universe