For night emh, all that happens is they start to scan magicbands and tickets to see if you are a resort guest for some of the rides.
As far as should you go? crowds can go any direction with EMH and the later emh is and the later you stay, the quieter it will get but it can be pretty packed at first, especially if it's the only park that's still opened and the park hoppers start to show up.. flip a coin, I've been at ones that were very empty and ones that were the worst crowd of the day.. you don't really know until you get there so have a very simple plan for what you want to do in emh hours and if it's less people you'll have some more freedom..