You can see the high fireworks from Boardwalk, but that's not the same thing as seeing IllumiNations. IllumiNations has multiple elements that are not fireworks -- there are fire barges and fountains, a large globe covered with projections, pyro on the water, light effects on the pavilions and lasers over the lagoon. None of those things can be seen from outside the park.
If it's just fireworks you're after, you can see them from Boardwalk. But if you want to see the show and hear the music and narration, you have to be inside Epcot.