I've seen them from Epcot, but just as very tiny flashes on the horizon. If you didn't know there were fireworks going on in that general direction, you might not even notice them. The Boardwalk and other Epcot resorts are several miles from MK, much too far to get a good view.
The Wishes show at MK is best seen from the park, but you can also get a good view from the GF marina, Poly beach, and the 4th floor observation deck at CR.
If you're dining in the MK area, there are good views to be had from Narcoossee's at GF, 'Ohana at the Poly, and California Grill at CR.