I think for a player to be considered great they would have won on various surfaces and over more than just a few years.
I think I read that the Australian Open used to be grass years ago.
A lot of player did not travel there then either .... cost?
And what about Nadal getting a Gold at Olympics and that not being figured in ... but it must have taken its toll physically and mentally.
What about players like the Williams Sisters who don't play in much of anything except Slams? Which amounts to resting up while others are defending their points.
It is a bit uneven as well that we now have something called the Open period when players like Pancho Gonzales beat just about everyone they played for years, up until he was 42 years old and the records really do not reflect that.