Depends. How many Aliens are there to help them get a foothold. Though, if it's any less than fifty, and the whole Planet knows, then yes, they are f**ked.
In my honest opinion, if a small pod containing no more than three Facehuggers were crashed on the Planet, without the Navi initially knowing it, it's goodbye to Pandora.
Then again, how powerful/big would a freakin' Navi Alien be?