We used to always go to MK for the last 15-30 minutes of EMHs on Christmas Day. It always worked, as it never got overly crowded until 10am.
This year we decided to go for the full hour, getting there for the rope drop show. We were able to go on Space Mountain, Buzz Lightyear, People Mover and Pirates of the Caribbean all before the park opened to the general public. About 10 minutes after the park opened we went on Haunted Mansion. Then we had FP+s for Jungle Cruise, Thunder Mountain and 7DMT.
Late-night EMHs are great for kiddie and family friendly rides like Peter Pan, Winnie, etc. Crowds will usually die down within the first hour of late-night EMHs. Peter Pan will have ~20 minute wait, Winnie will be walk-on.
Honestly, it's come to the point at Disney that there's no longer a "slow period" and there's never really a time that it's not crowded. It's just less crowded and less chaotic sometimes. You just need to plan your day and take advantage whenever possible. I certainly wouldn't avoid a park due to EMHs. What I would avoid is falling into the trap and being the person speed walking back to Frozen Ever After or 7DMT. Use those EMHs to target the second and third tier rides most guests will go to when they realize the most popular rides have a 2-hour wait.