The f- and h-pawns are the hardest pair to convert, because the defending king can sit on g7 and the pawns cannot protect each other. An active king and a far-advanced pawn still win.
Rook with f- and h-pawns against a rook is a famous problem case: the pawns are two files apart, so they cannot support each other, and the defending king often finds a fortress on g7 or h7. Here, however, White's king is in front of the pawns and Black's king is already on the back rank, which tips the balance.