It seems to me that aliens as an explanation for UFO sightings is inherently contradictory.
If you assume they came and are attempting stealth, then they've done a terrible job of what should have been a trivial task. Assuming they have the technology to detect intelligent life on our planet from many light-years away, one would think they could safely observe us from inside our solar system but outside the atmosphere, where they would be virtually impossible to detect. If they do need to enter our atmosphere, it should be easy to set some rules on movement (acceleration limits, etc) such that, at the very least, they do not immediately strike a casual observer as an unusual phenomenon. Instead, they have managed to be seen and recorded so many times that they have not only been sporadically observed, but have actually inserted themselves into our cultural zeitgeist.
On the other hand, if you assume they are here but do not care about being observed, one would expect much better documentation of them. For that matter, if they do not mind being observed one would expect they would hover or land near objects of interest: cities, planes, near world leaders, etc. Instead, UFOs always seem to have almost a furtiveness to them, quickly accelerating away when they realize they are being seen.