You're right mostly, there are also some special cases where it makes sense to hide the UFOs (or any radically new, advanced tech), some examples:

- the alien tech isn't really and/or fully available, they are half way in the reverse engineering efforts, and maybe other powers are in the race too

- the alien tech intrinsically reveals some currently hidden aspect of the universe/reality, if you present the technology publicly, it will be obvious that there's something new, and everybody will pursuit advances related to these new physics.

i.e. imagine discovering the eletricity in the XV century, it would have changed everything.