Lots of them are old school glass pane hoaxes.
Back then they didn't have CGI so guys like Adamski would stick model cut in half to a glass pane. Then they would move glass, or change angle. The way one can easily recognize "glass pane" hoax is they always avoid the outline of distant objects, like trees or roofs. The reason for that is simple, if the model overlaps a distant object it will be plainly seen that the object is not far in the distance, but closer to the camera.
Another way to recognize these hoaxes are angles. Humans subconsciously judge the distance to objects by object's roundness. A very close object would look extremely round to the human eye. Up to say about 300 ft 100m even if one moves his body just one or two steps or if, say, the object moves few meters, he can easily see that object's roundness would reveal new details or change the shape slightly or reveal some shadow. In these hoaxes, objects do move, but they don't reveal this roundness. These hoax models stay flat because they are most often just paper cut-outs.
Hoaxes are PITA because they make the study of UFOs harder. And we have so much useful stuff to learn out of these civilizations.