I know that different products work differently for each animal, and for my dog what works best is Biospot. Some animals do have allergic reactions to the spot-on treatments -- you just have to observe them very carefully when you first try a new product and be ready to throw them in the bathtub fast if they show any signs of a reaction.
Being in a sub-tropical climate, fleas are a big, year-round problem. I've used Biospot on the dog and both cats for years now, and for whatever reason, it's the only thing that works on them, or for the type of fleas they get, or whatever. I've tried all the big vet brands, and they didn't do anything at all for my animals. The Biospot keeps them completely flea-free, which is no small feat down here.
Just my experience!