Since I installed my 933L, I've always upgraded it via the MDL mobile app when directed or from the MDL web site on a browser. Even from work. Yes, I've been worried on occasion with using wireless upgrades. Has worked for me however. Wired is preferred though. However this does present a problem for those cameras mounted up out of reach or at remote locations where someone can't get to a camera readily.
One caveat with upgrading a camera over wireless is that the wireless signal between the camera and main host wifi router needs to be good and solid. Not environment interferences and other extraneous signals should be present. I think another issue maybe distance and range between the camera and main host wifi router. The greater the distance between them the lower the signal quality will be and thus probably be a cause of a failure to upgrade properly via wireless. My camera is well with in a 15' range from my router.
Also the configuration of the main host wifi router should be taken into consideration and what all is connected to the router at the time before attempting to update a camera. Possible other factors could contribute to bad upgrades happening over wireless.