Lusar / Cook:
UFO's are secret man-made vehicles originally developed from Nazi technology
UFO's are extraterrestrial vehicles based on known physics piloted by rational entities
UFO's are misinterpretations of known natural events or mental delusions
UFO's are manifestations a metaphysical multi-dimensional reality of which the human mind is a part
UFO's are manifestations of intelligences from a multi-dimensional reality with motivations beyond our understanding
The greatest skeptic, Menzel, tried to explain all UFO sightings by (often tortured) explanations that they were misperceptions of one or a series of natural phenomena. Those he could not explain in that way (close encounters, encounters with humanoids, delusions) he simply considered delusions of the human mind.
One step beyond this is the claim that UFO sightings are real, but they are inadvertent observations of tests of secret (mainly American) aircraft. These used jet and aerodynamic (Lusar) or antigravity zero-point technology (Cook) technology developed in the Second World War in Germany.
This is then followed by the conclusion that there are saucers, but they are alien technology, flown by visitors from other planets, following laws within our physics but that can still be deduced (Hill).
Vallee, after a long study of the topic, came to the conclusion that UFO's were not nuts-and-bolts hardware delivering visitors from other planets. Instead he found that they had much in common with creatures and events in mythology, including modern cults, religions, cryptozoology and psychic belief systems. He believed something was happening, but that it represented manifestations of a multidimensional universe that coexisted with our own.
From another direction comes legitimate scientific research into near death experiences. This has led to the observation that our DNA has insufficient information to encode the complexity of our bodies and our brains have insufficient capacity to store all of our memories. It has been theorized (see van Lommel and Berkovich) that a quantum field exists, outside of our body, which is the actual location of the design of our body and our consciousness and memory. Our DNA is only the "barcode" for our material selves, and our brain is like a television set, receiving the processed information and memories from our extended consciousness. When a television set is turned off, it does not mean that the transmission has stopped. Likewise, death does not mean the end of human consciousness, it is rather the end of the receiver in this space-time continuum.
It is a short step from this to the concept that our consciousness is part of an extended worldwide or universal or multidimensional consciousness, as advocated by moonwalker Edgar Mitchell and his Noetics institute. This is an individualistic expression of Princeton Physicist John A Wheeler's information universe.
This brings us to Keel, who like other paranormal or Fortean researchers, considered UFO's one of many manifestations of a multidimensional universe that the human mind is part of and can sometimes tune into.
And finally we are back to Menzel, who considered that inexplicable UFO sightings were human delusions confined to the mind. There is no real disagreement between him and Vallee or Keel - only in a view of what the mind is, what perception is, what reality is, how many co-located dimensions there are in the universe.
Of course there are mix-and-match versions. Obviously some UFO reports are of secret aircraft, although not necessarily any using alien or anti-gravity technology. The Roswell / Area 51 mythology posits that UFO's are both alien and earthly devices, derived from alien technology. Hynek seems to have been an agnostic, but it seems likely he favored Hill's conclusion rather than Vallee's. And it is possible that UFO sightings are a mix-and-match of all the explanations.
The point is, given the disarray of theoretical physics since Einstein, there is no objective way to differentiate between the conclusion that UFO close encounters are mental delusions, interdimensional visitors, or the product of an out-of-body extended consciousness. The only testable theory is Hill's extraterrestrial saucer hypothesis, and to date no public incontrovertible specimens of an extraterrestrial flying saucer have been obtained.