Archer is Shirou's own answer

Archer is the best antagonist Nasu has written, because he is Shirou's own answer. UBW's middle chapters are a debate between two versions of the same ideal — one that has not yet paid the cost, one that has paid it and is furious about it.

Heaven's Feel is where the book stops arguing about ideals and starts being about Sakura, whom the first two routes only glance at. If you stop reading at UBW you leave the book incomplete in a way that is hard to explain without reading HF — but the route is the book's final form, not an optional coda.