I'm sorry, but what does this sentence even mean, scientifically speaking? "Robust" means there's a lot of evidence, "nil" would mean absolutely no evidence. Are you saying there's "none" of a "lot of" evidence?
There's clearly a lot of some kinds of evidence that aliens may have visited Earth (all the testimony, photos and videos, radar readings, documented physical imprints and interactions in the surrounding environment, unidentified materials, etc.), which is why I assume this sentence is so convoluted.
If you're trying to say that the plenty of evidence there is isn't good enough, okay, I guess, but you should say it clearly. Nobody is saying that every UFO is aliens, or even most of them, but the gathered evidence in many cases specifically rules out just meteorites.