I haven't played many different courses, and none that are ranked among the greats, but I must be lucky, because I can't remember ever playing a course I found relentless. I usually play the right tees for me (mostly blue, sometimes white), and while I've certainly found some courses HARDER than others, I've never found one that didn't provide the occasional break and the occasional birdie chance (or at least the illusion of such a chance).
I think a truly relentless course -- for my level of skill -- would probably be a pretty dull one, with, say, most/all the greens perched up and bunkered, so not allowing me ever to try to run anything up there, whether out of imagination, fear or a flubbed shot. Dull in that it would play (and, just as importantly to me, it would look) the same for much of the 18 holes. Tighter fairways with lots of high rough or trees I can't really call relentless, because I can always choose to hit my 18 degree hybrid instead of a driver, and at least live to fight another day/shot.
Peter