You missed the point, it isn't about frequency or reliability. It's about what you can do if it fails. In my example above, several experienced people at the controle couldn't figure it out. A friend of mine recently had connection issues, messed around with the thing and eventually took the bike to the shop to get it rectified. I don't need this kind of stress during an ultra endurance ride. With a mechanical derailleur, all I need are spare cables (you bring them on 1000k and above, don't you?) and tools and I can fix 99% of any issue that comes up.
Likewise with tubeless. Sure they generally work. But on the rare occasion that they don't seal, I don't want to deal with putting a tube into a messy gunk-filled tire in the middle of an event, and also deal with mounting and unmounting tighter-than-usual tires in the process. And yes, a couple of people I have ridden with had exactly this problem - punctures that aren't sealing.