It is all based on the kid, I agree. We took my now teen and then preschooler to WDW. We were walking past Splash Mountain and his eyes lit up. We tried explaining that he was too small. To prove it, we walked by the height marker. Wouldn't you know, he was exactly the right height. We did a parent swap and let him ride twice. Highlight of his day!
The next day, we went to Hollywood Studios. We took him on the Great Movie Ride. He freaked out and buried himself in the foot well for the second half of the ride. We were struggling to get him on rides that day. So I kept telling him to remember that everything was pretend. He calmed down. We went to get pizza for lunch. Sitting down at lunch he looked at me and said "Mommy, you lied!" "You said everything here was fake, but this pizza is REAL."
So for my kid, roller coasters were fine, but the animatronics were problematic.