You can see the MK fireworks from various locations at all the monorail resorts; Grand Floridian, Polynesian, and the Contemporary. If you don’t have a MK view at these resorts you can see the fireworks from their beaches OR if you are dining at O’hana or California Grill they play the fireworks audio. Not sure if this happens at other monorail restaurants but when we ate at both those places we heard the audio during the show.
If you’re lucky enough to score a MK view room at the new Bay Lake Tower they play the audio on the in-room TVs, and perhaps the most excellent option is the lounge at the top of Bay Lake Tower; although they are limiting entry to DVC members staying at BLT.
As far as EPCOT fireworks go, I haven’t had too much luck. People say they sit on the Boardwalk and watch them, which is easily accesible if you are staying at the Beach Club, Swan & Dolphin, or the Boardwalk Resort, but when we did that there was LOTS of foliage obstructing the view. We even walked the Boardwalk trying to find a better spot and it seemed like no matter where we went it was fairly obsatructed & not so great.