While slope ratings are hardly perfect, at least in theory the relative difficulty/user friendly qualities of a golf course are taken into account and plugged into handicap calculations.
As a matter of psychology, it has often been my experience that guys who play hard courses - and invariably do not have the intellect to play from the correct tees - find it hard to break through to a really low score on a comparatively easier golf course. Their brains are not used to shooting five shots below their (stealing from David Owen) "Usual Game."
Years ago, I heard that Lanny Wadkins would sometimes play the regular markers at Preston Trail to get himself psychologically ready to go low. Yes, playing a cupcake most of the time and then suddenly getting horsewhipped at Oakmont is certainly a challenge, but in my view, the most important thing to keeping your game sharp is teeing it up at a variety of different courses.
The Creek Club in GA obviously carries a course rating too high relative to its resistance to scoring. By contrast, I've played courses like Del Rio CC in Brawley, CA that - though barely 6000 yards of dry fairways (par 70) - are maddeningly hard to undress. An average 3 handicapper can sometimes shoot between 70-74, but with tiny greens and a frequent desert wind, once things start to go sideways, it is easy to shoot 80 or more. Yet, the SCGA course rating is 67 with a slope of 115. In reality, a legit 3 anywhere else ends up carrying a 6 . . . . . . . which travels pretty damned well.