The FP+ for Illuminations just gets you into the roped off area behind the stores at the Showcase entrance/Port of Entry area. Otherwise, there is good viewing all around the lake, pull up a spot approx. 30 minutes ahead. Places to avoid are: any down wind direction, you will get the smoke from the lower fireworks effects blowing your way: the more crowded areas near the FP+ viewing area as those crowds are a huge mob of people leaving the park after the show: any roped off handicap viewing area, there is one next to the UK restaurant, not sure of any others: and any spots with an obvious obstruction, there are a few areas semi-blocked by trees.
Our preference is the far end of World Showcase. True, you won't see the lights on the areas you are facing away during the segment where they light up some countries, but it makes for a leisurely walk out of Epcot after the show to walk around the Showcase and exit behind the larger crowd. We often spend a while in Mousegears to let the crowd thin out too. Would rather stroll and shop than wait in a huge line for a bus or monorail, or in traffic in the parking lot.
We tried a FP+ for the Illuminations viewing last year and were underwhelmed. Much more of a crowd for leaving the park, and the view, though unobstructed, wasn't any better than others we've had around the Showcase.