I thought you were being objective at first but classifying 2015 WIM vs Fed and 2018 WIM vs Nadal as easy is absurd.
I'm not claiming each individual match was/is/would-be easy against those opponents. Only the 'type' of mactch-up it is before playing the matches.
For example:
For Nadal to beat Federer on HC/Grass is much harder than on clay.
For Djokovic to beat Nadal on HC/Grass is much easier than on clay.
For Murray to beat Djokovic in general is very difficult, but a little easier on grass bc he's 2-0 against Djokovic on grass (but too few grass matches to know for sure).
For Federer to beat Wawrinka on clay is more difficult than on HC/Grass.
For Federer to beat Nadal on clay is much, MUCH more difficult than on HC/Grass.
The above are general match-up difficulties by surface. This doesn't mean they will lose/win 100% of the matches if those surface match-ups are hard/easy, respectively.
Federer would rather play Wawrinka on HC than clay if he could choose. It would be 'easier' to get a win, but not always (he happens to be 100% on HC against Wawrinka but Stan has been close before, like Cincy2018).
Roddick is easy for Federer match-up wise, but he's even easier for Fed if on clay. Same player, one case is easy and the other case is even easier
.
General surface match-ups is what I said. And Murray is easiest for Djokovic out of Federer/Nadal/Murray in general, even though he's 0-2 on grass against Murray.
Surface match-ups is very important, not just their names.
Fed on clay is almost a guaranteed win for Nadal, but it's a completely different case on HC/grass.
And age is super important if the opponent is 30yrs+ instead of prime age of 20-29yrs.
