We went last year with a 2.5 year old. He enjoyed most of it. He fell asleep in the middle of it. This is a kid who normally does not fall asleep easily - and was incapable of napping in the stroller or my arms at Disney. He does have sensory issues but the noise of Fantastmic wasn't an issue. He was in my lap the entire time, but that's normal for him even without noise. My oldest also has sensory issues - to the extent that things like fireworks and even Lion King are hard for him to enjoy. But he loved Fantasmic both times he's seen it. We brought some earplugs just in case but didn't need them. I'm thinking that maybe it depends on where you sit.
We didn't do the ADR and waiting was not an issue for us. BUT we went in the off-season. It was the week before t-giving last year. It started at 6:30 and our plan was to get there at 5:30. That didn't happen - we got there closer to 6:00. We were sitting on the edge of the middle third about 3/4 of the way up since I worried about the intensity. There were even some seats left right before. Fantasmic was 5 or 6 days that week, with some having two showings but our night had just one. So, though I'm not going to say there won't be waiting, it is possible to not have to deal with waiting if you go the right time of year on the right day. The kids ate their dessert and played with their glow sticks so waiting wasn't that hard. (Glow sticks bought from dollar tree!)