I don't mean to be critical of Kemper at all, but from reading forums and from personal experience, the Kemper does seem to have a very slightly higher incidence of failure than other similar products I've seen. It's not really fair to compare Kemper reliability to tube amps, tube amps will always fail more as tubes are very fragile and amps are always taking a beating, plus new build tubes are not as rugged as the ones made in the 60s/70s.
Some of the Kemper failures I've read about are: the early LED issue, and distortion developing in the output circuitry, among a few other things. The first Kemper I had was a factory refurb model, and out of the box it had an intermittent distortion problem that required a return. But the replacement for that one (a new in box unit, not a refurb) has worked perfectly. From personal experience I have found Kemper's customer service to be outstanding in every way.
All that being said, the Kemper is a marvelously well engineered piece of gear, at least to my eyes (I have over 30 years of experience in engineering and manufacturing portable test equipment, so I work with this kind of thing daily). With all I've seen, I would not worry about field failures if the Kemper is treated with respect and its power is kept clean and well regulated. But you always need some sort of backup on a gig because any piece of gear can fail. And Kemper's excellent customer service and generous warranty should put your mind at ease, as much as it can being you are depending on a human built piece of equipment.