We have a disagreement between player and game master that I'm trying to settle: can the Backstab ability be used with a projectile attack? Rule as written doesn't include any restrictions on what sort of attacks can use Backstab ... and most (or all) rules about attack abilities state requirements/restrictions very clearly, so it seems that Backstab could be used with any attack from Advantage. Other opinion is that background text mentions 'blades' so Backstab is limited to blades so can't be used with ranged attacks ... and this strict reading would also disallow Backstab with hammer or staff or fists etc. And even axes don't stab.
My feeling is that the name of the ability is unfortunate (neither 'back' nor 'stab' are in the RAW, other than the title) and implies a major restriction on Backstab's use.
How should this be ruled?