We were at Disney 2/27 through 3/7 and here's what we experienced for EMH nights (copied from one of my earlier posts):
Evening EMH at AK: really nice, not crowded at all. Got to ride the safari in the cooler late afternoon hours (5:00 pm or so), and saw many more animals than I've ever seen previously. The park is really pretty lit up at night too. Of course, you can't see any animals once it gets dark, so you are limited at that point unless you want to hang out in DinoLand (which we DO NOT)!
Evening EMH at EPCOT: NEVER AGAIN!!!! I truly think it was more crowded that MK on a Saturday during the Holidays!! It was insane! The thing about EPCOT is that there is lots to do in the World Showcase even without an EMH bracelet, so the crowds never dispersed until after 10:00-11:00! Crazy.
Evening EMH at MK: Also crowded in the evening until Wishes at 9:00; once the fireworks were over and people realized they couldn't get onto any rides without a bracelet, the park cleared pretty fast. We had the best time at this EMH! We usually travel in the off-season, so MK is closed by 7:00 or 8:00. What fun to be riding tea cups or Dumbo at midnight!! We would definitely brave the early crowds to do this EMH again....but this is the only one.
Hope this helps!
Sammy