I think in this instance boon is used for its traditional definition as “a favor.” I think it is meant to be an unequal bargain that gives the Hag more than they return - so something like boosts to Attributes would probably be too much.
Ever since Paizo introduced rituals towards the tail end of 1e, I feel like they were looking for an excuse to bypass power limits when you are trying to do plot things like this.
The simple answer is that they borrow the extra power from elsewhere using a ritual- they have some powerful item, they perform it in a powerful location, or they have friends on the other side. Maybe some combination of all three, all boosting the hag's existing magical skills.
It is easier to work with if you put limits on this ritual's applications and effects. In the little mermaid example, you have a willing creature (or at least one that can be tricked into signing the dotted line), trading an ability, in return for a specific transmutation effect. And the price frustrates the very benefit they hoped to gain from the transmutation (ie- hard to romance a prince when you can't even say hello).
Going with the "Disney's the Frog Prince" example, you can have the hag incurring a debt, 'going into the red', by borrowing a greater power's abilities in order to enact a scheme. Whatever your hag offers the player is just a trap in order to get access to the payday (and the hag would have to pay dearly if the deal falls through).
In this way, the hag is just acting as the agent of the greater power in the mortal plane. So she might be weak, but it is more like the low level butler of a powerful noble being sent out to run errands. The party can beat up the 'butler' now... but you might want the 'noble' come look into the matter later on.