Butler National is, without question, the most difficult course in Chicago. As Ryan Potts noted, you can get on the bogey train on Medinah No. 3 (and I'd add Cog Hill No. 4 for that), but Butler can throw an 8 at you faster than you can think. The greens are small but oh-so subtly sectioned, the bunkers are unforgiving, the water is sneaky (the narrow creeks behind No. 11 and 18, as was shown), and while only No. 9 really brings trees into play (especially since the big oak on the 18th came down), the fairways aren't all that wide.
The USGA would bring an Open there in a second if the club was co-ed. And it will be someday, methinks. (There's plenty of room for tents on the polo field and the adjacent public Oak Brook Golf Course, and a deal could be made to shuttle fans in from nearby parking, maybe even the shopping center.)
The problem is the flooding. Butler's part of the designated Salt Creek flood plain, and while the 1987 flood, which forced the use of nine holes on Oak Brook to go with nine from Butler, is the most famous, the flooding happens more now than it used to, because the area to the north is more built up. An extensive pump system is used to control the release of water from Butler downstream to prevent more flooding to the south. And Butler (and OBGC) thus gets the worst of it. Lord knows what would happen if there were deluges within two weeks of a U.S. Open.