I've yet to see or hear of a dog drop dead from the wiff of an onion, clove of garlic, or vinegar.
The chemicals in many popular flea and tick medications and products, however...well, they have quite a different track record. Many of the synthetic chemicals used actually work because they interfere with the insects' nervous system. Either by blocking chlorine and therefore causing paralysis or by simply blocking nerve transmission completely. And given how many neurological symptoms have been documented and warned against while using these products...hm. I'll personally steer clear, and that is a decision I'm quite happy with.
Again, the dose makes the poison... so at very high levels you could see the same signs in dogs as in insects, but at the levels used in flea control products, there is no concern of neurotoxicosis from fipronil applied topically to dogs. Nor have I ever seen a confirmed, documented case report where someone can actually say "this happened to this specific person and this specific dog" beyond vague internet rumors of cases when the product was used topically as directed.
Also, the safety and side effect evaluations on a product like fipronil is performed on laboratory dogs fed large quantities of fipronil every day in safety trials, not studies in pet animals who have been exposed to the topical product on a monthly basis. The dose makes the poison.
There is nothing inherently dangerous about onions, garlic, or vinegar. If there were, I would think we would certainly know and plenty of dogs would be dead by now, given how many food companies put onions or garlic in their formulas.
Again, I don't actually care if you use Bug Off or Frontline or Bio Spot or a special rock you bought on Ebay that keeps fleas away. I just hate wooly logic and weasel words.