A lot of the cases have always been hoaxes or ignorance. Slow moving lights in the skies photographed by exited "believers".
The few remaining good cases are difficult to document. First of all, go out and take a picture of a bird. You get just one chance and it must be flying fast. Your result will not be very good. This our baseline.
Now look at the Nimitz incident. They measured a move of stationary at 80k feet to 80 feet in less than a second - something like 1200g, IIRC. 1200g is a downright silly number.
Now just take a good picture of this /s
The best current theory IMO is that this is done my manipulating/creating geodesic lines. This will create "gravity lensing" (like Einstein crosses) further blurring the craft.
Anyone serious but curios should take a look at uaptheory.com - it is good work on display by someone who understands physics and relativity.
We won't get a good picture because they move ridiculously fast.