Last year was our summer of fleas. Vets said frontline no use, it’s too weak. The drug companies have to advise a huge safety margin, partly to avoid litigation and partly because the dose has to treat a weight range, so eg., the 10kg dog gets the same dose as a 17kg one.
I reluctantly accepted a prescription of Simparica monthly tablets, google has all the hysterical warnings , but the vet said he’d never had any complaints. It worked perfectly, and room fogger sprays cleared the house.
My sons dog has Bravecto, three monthly, again no issues. Personally I didn’t want such a long lasting drug.
The pain is that you have to pay for a vet appointment as well as the tablets for a check up to assess if the pet is in good heath. You mention possible pancreatitis, but the adverse side effects for these are likely to be neurological, not digestive.