Bullshit. I've got Crohn's, and have had more colonoscopies than I can count since age 13. About 75% of the time, I'm throwing up due to the prep. The prep is the WORST part.
It has gotten better over the years. If you go for one, do NOT let them give you the jug full of powder. I've had much better luck with prep-o-pik.
The other PITA is they generally won't let you go for the procedure by yourself, since you're under anesthetic and don't trust you to get home OK. That means you have to find a friend who is willing to wait 2+ hours for you.