The fun continues at aboutSETI.com:
The Semantics of Alien Visitation
In addition, both Keel and Vallee share a pronounced distaste for tales of crashed alien hardware a la Roswell; according to Keel's take, a physical event as pronounced as a crippled UFO is effectively impossible, as his "aliens" are largely immune to dangers posed by the world of gross matter.
Some speculators make the mistake of thinking that the revisionist, postmodern cosmology advanced by Vallee and Keel is incompatible with the hallowed ETH. But in truth both models are anything but mutually exclusive. It's even possible to imagine technological aliens from some distant star developing the ability to access Keel's "superspectrum" for reasons we can only guess.