As an example, we where there just last week.
The parks were VERY BUSY during the day (weather was hot and HUMID) and we chose to go back to our resort for an afternoon nap (in the AC), then go back to the parks after dark.
On Sunday, May 21 (MK Evening EMH, 10pm-1am) knowing the reputation for MK evening EMH's to be OVER-CROWDED, we watched the 10pm "Wishes" from the beach at the Poly, then rode the monorail over to MK thinking we would not get to ride much, but we could at least enjoy the night-time atmosphere.
Well, the MK was nearly DESERTED for the whole rest of the 3 hours we where there,
Walked onto Jungle Cruise, Aladdin's Carpets, Country Bears.
Rode Splash and BTMRR over and over (we even had a Dole Whip with very few guests waiting to buy them.)
Maybe the crowds were heavier over at Fantasyland and Tomorrowland, but we had ZERO problems where we were.
We left the park very happy (and tired, of course) at about 1:15am!
So, again, it doesn't matter what MIGHT have happened in the past... NOBODY can predict exactly what an ever-changing universe of guests WILL or WON'T choose to do on a given EMH day/night.
EMH's are FREE to WDW resort guests... why not go and see for yourself.