Sure, Nadal has won 18 slams, including golden slam, so no sane person should trash him. But OP is asking an interesting question (although in a bit trashing way). When a player wins FO, loses R1 at W and then wins USO, its natural to ask why? Between 2011 and 2018 his grass results are surprisingly weak compared to his overall results.
Also no trashing here...
Fed again has a ratio of games to points that is 1.86. You can check it out yourself, but you need exact numbers. Take % of games and points. Subtract 50 from each. Then divide.
1.9 is fine for grass, but around 2.25 or so is good on clay. Why?
I have NO idea. Clay is the surface where players win the most games, grass the one with the least. It's all about margins. so when you have % for all games for careers, they run a few % points higher on clay. The margins are smaller. HC is in the middle.
The ratio is highest on clay, lowest on grass, in the middle on HCs.
That means that when looking at games and points, around 2.25 or anything close to that is great. 1.86 is very low, and Fed has been about there for his entire career. Since his total game% on clay is noticeably weaker when you would expect it to be higher, it's very plain he has a weakness on clay. I'll let other people try to figure out why.
So I don't see that Fed is any better on extremely slow surfaces than Nadal is on very fast ones, but the way the tour works it's a lot harder to skip grass when Wimbledon has such a huge rep. If Nadal had just won his 12th Wimbledon, most likely he would be the favorite in any GOAT debate., and that's probably a huge understatement. The biggest reason why Borg is so hyped in spite of his very short career is that he had a Nadal-like dominance on clay but also those 5 straight Wimbledon's. He started out his career as a famous clay grinder, but once he had those grass credentials it was tough not to put him up very high.
I don't know why Nadal has been so vulnerable to early losses on grass. He looked like he would do well after 2008, but since then it's been tough. Some say it is because grass bounces so much lower the first week when it is still green, and there are other reasons. Personally I think it is at least 50% mental now. His knees hurt, he knows he's expected to do well, and his losses are in his head. So maybe once he gets past the first week he starts to relax and begins to believe he can win again. The biggest head-scratcher for me is Djokovic. I expect him to win on HC, but not on grass. Why he has had so much more success there than Lendl has never made sense to me.