The accompanying link to “Hard to Watch Films” barely scratches the surface of what we mean by the terms “challenging” and “difficult” or the coinage “hard to watch films”. The link seems to think “hard to watch” most often means graphic and disturbing content. It also categorizes some of the films as hard to watch because of their verisimilitude (Funny Games ) and true accounts (Elephant Man ), but both these categories of difficult film are still part of the overarching graphic or disturbing content films.
I appreciate Vivian Sobchack’s attempt (Film Comment, Jan/Feb 2014: 50) to categorize the varieties of difficult film:
There are those that are difficult to watch because of their explicit violence or graphic sex (Gaspar Noé’s Irreversible), or their extremely disturbing visceral effects (Kirby Dick’s documentary Sick: The Life and Death of Bob Flanagan, Supermasochist). There are also those that are pleasurable in their difficulty: cerebral “puzzle” films with intricate plots and/or structures that require some effort to figure out (Christopher Nolan’s Memento, or Carruth’s debut feature Primer). Then there are films that are difficult because they push the limits of representation as far as it will go (Charlie Kaufman’s Synecdoche, New York and its mise en abyme of infinite representational regress). There are others that go even further and push the very limits of cinema itself (Derek Jarman’s monochromatic Blue, or Michael Snow’s La Région centrale). Most commonplace, however, are those films that are difficult to watch because they push nothing: they’re unchallenging spectacles devoid of thought, affect, and any reason, other than mercenary, for being at all (certain kinds of mainstream trash like… Hansel and Gretel: Witch Hunters).
The most difficult films, for Sobchack, are those that refuse even these conventional difficulties. These works transgress narrative logic or cinematic specificity and “make sense” to us, “sensuously, experientially, in the phenomenological ‘now’ of seeing, hearing, and touching (if always also at a distance)” (51). It is inappropriate to decipher one of these films, nor be simply shocked by their displays of bodies. Sobchack argues for the importance of “meaningfulness” in the sense of “being present to” an object, thing, person, or for our purposes here, a screen and its images.
Shane Carruth’s Upstream Color (2013) and Terrence Malick’s To the Wonder (2012) are Sobchack’s recent examples. In my current work I’m thinking through Carlos Reygadas’s Post Tenebras Lux (2012) as a similar kind of difficult film.
 I nearly typed irritating plots, a clear indication of my film preferences.