Federer's slices aren't all safe though, sometimes they're tough shots to pull off, especially the short slice which everyone complimented long ago. They used to call it one of the unique things about him, because no one else could really pull a short cross court slice as consistently as he could. It was a shot the entire tour had to adjust to get used to, if they wanted to win. No one else really does it.
But I agree in the sense that he didn't use the 90 sq in racket because he wanted it to be harder or anything. Its just what it is. That 90 sq in was an advantage at one point. Only reason it became a disadvantage is because he used it for so long.
To me, its not that all he does is harder. To me, its that all he does is pretty unique and an indicator of intelligence behind his game. He credits a lot of his technique to his coach from when he was growing up. When I watch his game, I don't look at the prettiness of it. I see the brutal efficiency.
I think he used the 85sqi PS for at least 4-5 years since his debut.
Then he switched to the 90sqi around 2002-2003.