Hi,
EVERYTHING is very packed on New Year's Eve...often times some of the parks (but usually not Epcot) will close because they have reached capacity.
If you do go to a park on NYE I suggest you don't leave at any point to take a break, because you may not be able to get back in.
But if you manage your expectations, make ADRs, and use FastPass+ wisely you will have a wonderful time!