Much like Cybertronians, the threat with them is that they can hide in plain sight without you knowing any better. Unlike them however, Yeerks are breeding in vast numbers, their reproductive cycle always makes more than replacement-rate so there's a constant need for new hosts. That's why Earth was such a great find for them, when they were on their homeworld they were stuck with the semi-sapiant Gedd hosts who were basically down-syndrome: the species. They got Taxxons, giant cannibal worm monsters, then got their first good bipedal hosts, the Hork Bajir, nearly exterminated by Andalite bio-warfare. Then they found Earth with our 7+billion member species and opposable thumbs and so they were like "fuck yeah these things are perfect".