Yes, EMH days are busier than non-EMH days.
That being said, if you're like us and only spend a few hours max in the park then going to a morning EMH can work well as long as you're out of the park before lunch. Lots of people will go to the EMH park even if they're nowhere near making it to the park in time for EMH. The flood of people will generally get bad somewhere around 11am. We've done MK morning EMH a couple times and it works perfectly with a late breakfast ADR at 'Ohana or 1900 Park Fare since the ADR forces us to leave and the restaurant is starting to thin out some as our meal progresses since we've got one of the last ADRs. It just works nicely for us. Evening EMH only works well if you'll actually stay to the end of EMH and if it's a really late one (as in until maybe 2am) otherwise the crowds of people who take in the evening EMH will be bad.
I really like using the crowd calendars over at easywdw.com. Josh does a really good job of listing the parks and which will be best vs worst as well as explaining the reasons. He also explains things like suggestions for if you really want to take in an EMH despite the park being listed as super busy.