My justification is the Hound could easily block his brother’s swings during the 1st season episode. I don’t see Brienne being able to stand up like that to the Mountain.
The outcome of the Hound/ Mountain is in doubt because the fight was stopped but you have a valid point.
Again, if the show had followed the book you would have seen Brienne climbing a cliff and throwing boulders down to sink ships that were pursuing her and Jamie. So she’s plenty tough.