UFOs and intelligent alien life - to the extent they are not mystical or supernatural phenomena - are susceptible to the scientific method, at least putatively.

UFOs exist. You see something in the sky. You don't know what it is, therefore it is a UFO.

The possibility of intelligent alien life is distinct from UFOs. The Drake equation is - to my mind - just a back of the envelop type of computation. Rather, the equation is probably valid as a model, but the inputs are just guesses. So maybe it's like a Fermi problem - but still a string of wild guesses.

The existence of intelligent aliens is separate from the question of UFOs. The existence of aliens is separate from whether they might ever have visited the Earth.

While I consider the idea that aliens have visited the earth silly without some compelling evidence to the contrary, I don't think all claims for unlikely phenomena are equally unlikely.

If Bigfoot and Loch Ness were not hoaxes, they wouldn't require science to be rewritten. These are very unlikely, but not utterly ridiculous. Well, at least they are not AS ridiculous as, say, alien visitors - which are an order of magnitude more unlikely. Even more unlikely than that would be something like ESP or remote viewing.

To summarize: there are a lot of absurd ideas, but not all absurd ideas are equally absurd.