I woke in the middle of my first one due to inadequate sedation and it felt like someone was pushing their fist into my stomach too hard and/or having cramps. Tolerable but unpleasant. I elected for propofol on my second and was happier (though both midazolam/fentanyl and propofol basically make you kinda useless for the rest of the day).
my guess is that they take more care when they know you are not sedated. I know a guy whose intestines have been perforated during a colonoscopy if he wouldn't have been sedated he would have felt the perforation right away