Life on other planets in our universe is not against doctrine.
You can still use your own reasoning to determine credibility. Where I live, on a reservation, there was an old woman who gave a story about a saucer stealing her cows, and a ladder came down and person started climbing down it, and she got scared and left. Every ounce of the tale reeked of nonsense of course.
My private opinion - the one that makes the most sense to me - is that the most common reason people tell these stories is because of a deep, disordered, inner craving to feel special, or unique, or set-apart, and since they cannot find that naturally, they invent a reason. I believe this is the same reason people sometimes call themselves prophets, or that they have memories of being somebody famous in another life, or that they believe in a grand conspiracy, or any number of other things. It's about making themselves feel special, which gives them a rush, not unlike drunkenness, heroine, pornography, explicit romance novel for women, one night stands, etc.
I doubt UFO sightings are demonic. I think it's more about psychological instability, poor self-esteem, and a craving for vainglory. Giving false testimony is sinful but the extent of the sin depends on the circumstances of each person.
Be patient and charitable.