The reason it was a nightmare is the last time we had a table next to the tank, our table was overrun by other guests who wanted to look at the tank. I am not a fan of people yelling and screaming in my ear back to their table, peoples hands all over my table when I am trying to eat, being coughed on, etc. And it wasn't just kids. It just made it a really unpleasant experience. Maybe it doesn't happen often, but I doubt our experience was the only time.
It's not easy seeing IllumiNations from inside LH. If you don't get a window seat, it isn't really worth it, IMO, especially if its your only night in EC. People who get a seat at the window tend to camp out, so you need a 7:15 or so ADR in order to get one of those tables, and even then it's all luck (you can't request a window seat). The only restaurant you could book and guarantee a spot for IllumiNations is Rose and Crown in the UK. They have some tables that overlook the lagoon but also have a standing area to watch from and the view is pretty good. There are a ton of great places around the World Showcase Lagoon to watch from. You are better off with a 5:45-6 ADR anywhere and then look for a spot when you are done. The only thing you'll really be closer to in either of the Mexico restaurants is the exit at the end of the night, but we much prefer to take our time leaving and walk around the World Showcase a little to let the lines at the bus die down.