I think for ANY product out there, if it's strong enough to kill a bug, it's strong enough to potentially cause other problems. Doesn't matter if it's 'natural' or chemical, or whatever, if it kills bugs of any type then it's got potential for bad, especially if not used properly.

I've been working at the vet hospital for a long time, I don't know how many thousands of cats/dogs we've used flea products on, but in my experience Frontline Plus and Revolution are two of the safest that we've seen far less reactions or problems with than the others. I personally have used both products on my cats without hesitation and will continue to do so without fear.