I've not read the Endless Love novel, but the film isn't great, I quite agree. Also I agree on Kubrick. His versions of all the novels he adapted are singular and brilliant in their own right. The TV series version of The Shining, which is more faithful to the novel, is utterly forgettable compared to Kubrick's iconic horror masterpiece. A Clockwork Orange has important tweaks too, as you rightly point out, as did Barry Lyndon (a much underrated film in my opinion).