Interesting video.
I have long had some questions that I’ve been wondering about on this topic.
Can these craft be observed on radar, and under what circumstances? (It seems some are and some aren’t)
What makes them crash?
Yes, there are very good instances of radar tracking. One was quite a sensation since the FAA head even brought the tapes forward after the CIA thought it had taken the originals ... they collected copies not realizing or even asking the FAA director if the ones they confiscated were the only copies. That particular incident happened to a Japanese airline pilot and his sighting, which was well documented with radar imagery and the actually recordings between ground and the pilot.
BTW, that FAA retired director testified at the 2013 Citizen’s Hearing in June. As to why they crash, apparently our heavy radar pulses just aftere the war, around our only atomic bomb wing in New Mexico was instrumental in that Roswell incident, an incident which had two crash sites, which most folks do not realize, when two UFOs collided, with one being torn apart and the other crash landing where a college archaeological class happened upon the scene.