I have a broader take on this. The more segmented and/or slick the greens are, the more pressure is placed on my approach shots. The farther my approach shots end up, the more putts I usually take. Like a majority of golfers, I do not possess the ability to stop my approach shots wihth-in a 3' dia of where they land, so the more slopey or undulating green are, the more afffect they will have on my run-out on my approach. Since Oakmont's have been mentioned several times here, the slickness of those greens forced me to play a completely different approach (because they scared the hell out of me). Therefore, it is my contention that the level of induced difficulty extends beyond just putting once you are on the greens, but rather to the approach as well.
So, I'll line up with Archie (and Jay F).
Also, I think some parameters on just what constitutes "Hard" are needed. Are Big 3-D undulations "harder" than say steep 2-D cross-slopes. Green size, and speed along with receptiveness (how firm) the green is are also factors. Those Oakmont greens got in my head because not only were they very quick (11-12+), they were very firm and large. Talk about confidence killers!