I'm in the camp that it's harder to run the gauntlet now rather then before. The consensus of almost all players and commentators is that the depth and overall level of play in the field has improved.
While some of the differences between surfaces has been blunted (in a good way IMO for quality of play), I find it inaccurate to submit that the guys are playing exactly the same on all surfaces. You can't watch the FO and Wimbledon back to back and make the argument that it's the same. The serve is still a bigger weapon on grass, points are shorter, positioning is different, players are attacking more (although not with serve and volley). FWIW grass court tennis is actually the most enjoyable surface to watch these days as you get incredible points, it's 1000% better then a 1990's ace-fest
I don't think there's ever been a time where a player might have to negotiate the individual excellence of their peers that was harder then going through (Fed, Novak, Nadal, and Murray) to win a calender slam.