If they were using generation ships, we would definitely see them because there is no stealth in space and they would have to have a visible heat signature, and they wouldn't be hovering and zipping around the way UFOs are reported to.
If we have to take UFO reports at their word, just for the sake of argument, what is described at the very least appears to have some form of non-reactive propulsion that acts like gravity control. Just them being able to move at high velocities without creating a sonic boom or any visible means of propulsion would put them in the realm of using some kind of weird new physics we don't understand.
"... being able to move at high velocities without creating a sonic boom or any visible means of propulsion"
maybe you're looking a vessel with more than 3 dimensions, and when it moves throught the next dimension/s, it allows it to do those kinds of movements. Maybe we're just seeing just an "expression" of a 4th-5th dimension vessel in 3D, but it's not really fully interacting with our three dimensions (maybe it's just leaking photons), and that is why those things can make impossible things like instant acceleration, submerging in the ocean at high velocity without making a minimal splash (maybe they could "submerge" into solid ground as well? that would explain UFOs in mountains, if you can get into a mountain to hide your presence, mountains look amazing as amazing parking lots for UFOs).
maybe if the universe actually has more dimensions than the three we can perceive.
"surpassing the speed of light" in 3D, but done in 4-5D is just a matter of traveling a lot less "space", hence you actually didn't need to surpass the speed of light, but you travelled "faster than light", by travelling across the 4th-5th dimension.
Travelling like that could be just like walking 100 yards in the 5th dimension, but in 3D you travelled 100 light years in a matter of minutes (of walking).
Even if something like an Alcubierre drive were possible, I wouldn't be surprised if there turned out to be some kind of catch that still limited you to the speed of light.
If we have to take UFO reports at their word, just for the sake of argument, what is described at the very least appears to have some form of non-reactive propulsion that acts like gravity control. Just them being able to move at high velocities without creating a sonic boom or any visible means of propulsion would put them in the realm of using some kind of weird new physics we don't understand.