Generalized questions get generalized answers.

OP - The best answer is to read the fare rules for the specific ticket you propose to purchase. Fully refundable means just that. E.g., you get a refund to your original form of payment without any fees or penalties of any kind. If there are fees or penalties, what are they?

Also remember, that if you book through a third-party such as one of those websites that call themselves a TA, you are also subject to the third-party's rules over and above the carrier's. It is possible to purchase a fully refundable ticket through one of these third parties and have the third party impose fees.