I'm fine on Incredicoaster until the very end
This was my issue too! I think it had to do with the way it decelerates so rapidly. It was like my 'vision' couldn't stop as fast as my body did. My brain was telling my eyes I was still moving and simultaneously telling my body it had I'd stopped moving (or vice versa), which caused a sort of disorientation that resulted in nausea. I had the same thing on Everest. The Scopolamine patch removed 99% of that feeling on Everest which made it 'tolerable'. But even so, chances are I'll skip Everest next year.
I may be willing to try Incredicoaster next time I'm at DL, but I'm going to time it very carefully. Maybe make it the last ride of the night so if I do get nauseous it won't ruin the rest of the day.