Birds and the kinds of small lizards and amphibians that cats hunt are very active hunters of insects, keeping control of that kind of pest. Cats also like to bury their waste and that nice soft garden soil could look very tempting. It's something I've seen people complain about on other
gardening forums.
On the other hand, I have two cats and there are numerous other cats that roam my neighborhood. I'm fairly sure they're responsible for keeping the squirrels mostly out of our pecan tree. The worst I've ever actually seen them do the garden is leave foot prints in freshly planted beds.
If rodents are a big part of your pest problem, then I'd be very grateful to have a nearby colony of feral cats. Just be aware that cats will reproduce to the maximum amount possible based on the
local resources. If you have plans to
feed them it would probably be good idea to see if your area has any of the catch and release programs for sterilizing feral cats.