Very uncontroversial opinion, and probably carries extra weight given who it is coming from. Nadal saw more of the best players of this generation up close in big moments than anyone. He is not speaking with outsized authority. He is an authority.
No one got a closer view of Federer in full flight at the height of his powers than Nadal. When Federer was at his peak, Nadal was the only player who could consistently push Federer far enough to expose the full spectrum of his game and see how he reacted when challenged up close.
Nadal literally bullied Federer till 2017. Apart from a few early matches and some hard court battles, result was almost for everyone to see.
It was some 23-9 or something - their h2h before 2017. Lol for a rivalry.
Rivalry is where things are even and not so skewed.