I've been running ultras since 2008 and have rarely heard of instances of cheating, although I'm not an RD either so haven't been in a position to verify the results. Yes, it does occur, but to infer that "many" people are cheaters in the sport is off-base and disparaging to the great majority of us who don't cheat. I would venture to say that cheating is more rampant at the marathon distance because the recognition is greater among the general public...ultras are still largely unknown to most people and somewhat of a fringe sport.
I ran Comrades two weeks ago. Page 42 of the race program is titled "Comrades Issues Warning to Potential Cheats." The secondary headline is "This year, thanks to the tip-offs, we have some names which we will watch carefully. If we catch them and they are found guilty, we will name and shame them." 9 paragraphs of detailed warnings. Cheating is obviously a rampant problem in the greatest of the ultras.