To clarify, the FDA didn't issue an actual warning, but rather an alert. This is because the drug label hadn't previously included some potential neurological side effects which reports were being made on. The manufacturers are to add these potential side effects on the warning label. I don't know if this has been done yet. The FDA considers the drugs safe but do recommend speaking with your vet about all potential side effects, especially in relation to your pets general health history.
The FDA's role is to collect data from vets, clinics, and owners and issue guidance based on that data.
This is the link to the FDA requesting label information changes by the manufacturers:
FDA alerts about potential for neurologic adverse events in dogs and cats when treated with isoxazoline flea/tick drugs Bravecto, Credelio, Nexgard, Simparica.
www.fda.gov
This is a link to the FDA's Electronic Reading Room with info on a number of complaints made on various products. The isoxazoline products data is listed toward the bottom under Safety and Health:
Frequently requested records from the Center for Veterinary Medicine.
www.fda.gov
This is the link to the collected data for Simparica:
This last link is strictly an example since it's for reports of DCM but this is the way the FDA gathers the data that they then issue statements on.