Feral cats can spread parasites and disease to other pets.
(and to humans).
They also decimate the bird population (and hey, yes, we need birds to eat insects).
I’ve never caught any disease from an animal. I have caught lots of diseases from humans.
Meh.
They keep the coyotes, racoons, owls, and hawks fed.
So I guess it depends entirely on where you are.
Around here cats are pretty low on the food chain, feral or otherwise.