On the films that are difficult to recommend because they require more preparation than most people will do.
There is a category of film that is genuinely accessible — not slow, not formally challenging, not long — but that requires biographical context to land with its full force. Without the context, the film is competent and unremarkable. With the context, it's devastating.
The prime example is Hirokazu Kore-eda's Nobody Knows (2004), which is based on the real story of four children left alone in a Tokyo apartment for months when their mother abandons them. Kore-eda found the actual children and used them as performers. Knowing this changes every frame.
But this creates a recommendation problem: if you tell people the context before they watch, you've loaded the experience in a way that forecloses their initial response. If you don't tell them, some people will watch it as a mediocre drama about neglect and miss what it actually is.
The solution I've arrived at is to say: watch it, then read about how it was made. The second viewing with the context is the intended experience.