Complex sentence: Nowadays, the term is more often applied to those cases that are believed by some to be the spaceships of extraterrestrial aliens, although no persuasive evidence has ever shown an identified flying object to be an alien spacecraft.
Split into simpler sentences: Nowadays, the term is more often applied to those cases that are believed by some to be the spaceships of extraterrestrial aliens. This, however, is debatable, since no persuasive evidence has ever shown an Identified Flying Object to be an alien spacecraft.