This is a VERY individual question insofar as responses are going to be based on how they go about navigating their way around a golf course.
That is because there are continuing themes woven into the work of Nicklaus, Rees Jones, Dye, Tom Doak and certainly C&C.
The individual "skill set" required to play their designs is very different for each of these men - and those similarities from course to course are a common thread throughout their body of work.
A Monet always looks like a Monet, just as it only takes a single note from his guitar to identify David Gilmour.
Some skill sets require purely physical ability and a well functioning left-brain approach to golf and life (read: Nicklaus), while others depend on right-brain visualization and "outside-the-box" creativity (read: Doak).
I've friends who waltz around Nicklaus courses, barely breaking a sweat, who love every hole. Now, take this same gunner to Pacific Dunes with blind shots and bunkers in the middle of the fairway and they hate it.
PGA West from appropriate tees is not that hard of a golf course. I have played it plenty of times. It gets that reputation from dullards and fools who play from the Black Tees and cannot figure out why they need a tourniquet for their necks by 3rd hole.
Pete Dye depends on psychological terror, but when you come down to it, if you can block out the monster and swing the golf club, the shot is never terribly difficult.
A Nicklaus course nearly always (Mayacama excluded) requires a shot I do not have in the bag. I never have and never will. Thus, it is like getting horsewhipped 18 times in a row by a sadistic prison guard.
The problem with Nicklaus is too often there is no place to miss - to cheat it a bit over there for a safe par. No options, no choices, just pound pound pound hole after hole after holes, course after course after course. I've played 25 or 30 of them - don't try and argue with me.
Rees . . . . no point in going there. Some people like Lake Merced. We can agree to disagree.
"Hard" is only in context of what shots you have in your bag. Ran's concept of "inspiration," that force of aesthetic and strategic appreciation that makes your heart sing with joy - and elevates your game to meet challenges is another subject altogether.
There is nothing harder for me to do than keep my concentration while being subjected to a repetitious obstacle course - yet take those same obstacles and present them to me in a strategic context, then they do not seem so insurmountable.
If I hate the architecture, the course is too hard, even when it is not.