Kris:
I think Erin Hills has the potential to play very tough -- but, the key word is potential. Re. the greens, with only a few exceptions (notably #3, and perhaps #2, which is a minature version of a Pinehurst #2 green), I didn't see any greens at EHills that rival the slopes and contours you find at the Blackwolf Run courses. To me, that means they'll have to really push the green speeds at EHills to make them a challenge -- a lot of the greens there are medium-sized, with two distinct tiers.
But if EHills plays fast-and-firm, I can see folks have trouble out there, because the traps there are really menacing and difficult, and getting a ball into the fescue will usually lead to a stroke loss. One of the things about EHills -- noted by others, but worth repeating -- is the great elasticity of the course; a number of holes can be set up to play at a variety of yardages, and you can easily have specific holes playing at 100 yards difference on different days of the tournament. That can lead, I think, to golfers being uncomfortable there, because they can't attack the hole the same way day after day.
I think one of the real interesting things about the Amateur is the difference between the two qualifying courses -- EHills and Blue Mound. I can't really think of two courses that are at wider ends of the golfing spectrum in the demands they make on the player.