Even before the advent of recent negative developments in the Roswell case, I have always felt that a UFO would never crash. However, because of the impressive witness testimony about which I was told, I suspended judgment and allowed for the possibility that Roswell might be an exception -- some kind of one-in-a-quintillion fluke. That was, in retrospect, a mistake.
The problem with the concept of a UFO crashing is that as technology advances, so does reliability. Be it with cars, airplanes, televisions, or wristwatches, the reliability of today's technology far exceeds that of the technology of just a few decades ago. For example, because of the high reliability of their engines, long-range, twin-engine commercial jetliners are now authorized to fly nonstop across the North Atlantic. A few decades ago, that would have been unthinkable. (The positive correlation between advancing technology and reliability applies to "proven" technology, not experimental state-of-the art machines still in the developmental phase, such as experimental aircraft or space vehicles.)
With today's industry-average engine-failure rate of less than one failure per 100,000 flight hours, the chances of both engines of a two-engine jetliner failing during a given hour of flight are less than one out of 10 billion. Figuring 50,000 aircraft-ocean crossings per year, and factoring in such variables as average time over the water and average distance from land, the odds are less than fifty-fifty of a double-engine failure and consequent ditching in the North Atlantic of even one such aircraft over the next 10,000 years.
This incredible degree of reliability is found with a technology that would be primitive compared with a UFO. Even with today's relatively "primitive" technology, our commercial aircraft have very efficient collision avoidance systems, as well as excellent radar systems for avoiding thunderstorms and their associated hail and lightning (phenomena, incidentally, that are surely not unique to this planet).
If we assume that UFOs are extraterrestrial spacecraft and that some of the many reported UFO sightings are genuine UFOs, we are dealing with machines apparently capable of high-speed right-angle turns, incredible accelerations and speeds, and wingless flight -- not to mention of traveling light-years through the void of empty space in, presumably, a relatively short period of time. Such capability would require a technology totally beyond our present understanding of physics -- a technology the sophistication of which we cannot even begin to imagine.
Because of the positive correlation between technology and reliability, such incredibly advanced technology would most certainly mean a correspondingly high degree of reliability. Common sense dictates that the chances of such machines crashing, breaking down, or colliding would be all but zero. It certainly would be many orders of magnitude less than the already infinitesimally small chance of one of today's twin-engine jetliners having a double-engine failure.