My aunt had her intestines accidently perforated by one of these procedures. The result was that she nearly bled to death, and took 3 months to recover.

They have it now so that you can poop in a cup and they can scan it for cancer cells. Nobody needs a colonoscopy unless they have ongoing intestinal health issues that they have to examine. There is no "routine" procedure that could result in nearly killing you, even if it's a small chance.