But all players in any era access to the same equipment, it's not like one has the advanced tennis racket/string over his peers.
That's certainly true. The point I was making, however, is that in the 1990s, mastering all aspects of tennis in order to dominate, was harder than what it is today. This was the case because of the different strings, and that this in turn meant more variations in playing styles back then. Sampras was an aggressive, attacking player, who took risks. On clay, that sort of an approach was more of a disadvantage, and other players were better at the grinding, more patient, style of tennis.
In today's tennis, it's all from the baseline for the most part. The variation on different surfaces is based on things like standing closer to the baseline on grass, putting more emphasis on your serve on hardcourt/grass, while being more patient and more of a grinder on clay. In the 1990s, there were a lot of variations in playing styles on top of this, precisely because gut strings meant more flatter balls being hit during rallies, and thus a greater chance of unforced errors being made, meaning variation in style.
The improved string technology today has meant that tennis' variation in styles has gone well down, meaning it's easier to dominate tennis by mastering the baseline game. I think this era is tailor made for a young Muster, without so many net rushers.
You can't judge which era is more tougher based on technology. Today's players hit more power/pace than the past generations, except that the courts are now sandy and slow which compensate the power/pace.
There's no carpet courts today, as they were fazed out slowly over a decade from 1997 onwards. But contrary to a lot of people's beliefs, the different string technology, is a far greater factor in tennis play predominating around the baseline today than any surface speed. Poly strings have a far greater tendency towards topspin balls, meaning more margin for error during rallies, while having greater power. As a result, players are forced back to the baseline, and play predominates in the back court.
If you watch tennis in the 1990s, it clearly has less power, but the gut strings have a far greater tendency towards flatter balls, meaning less margin for error during rallies. This means, compared to today, that the chances of making unforced errors by rallying from the baseline were much higher. It was also harder to dictate during rallies with the necessary power and authority. Therefore, there is greater variation in playing styles in the 1990s. Therefore, tennis was a harder sport to master so completely in the 1990s.
Volley, and net-rush is rare because the ball bounce high, which makes it harder than in the 90s which the conditions are tailored-made for serve/volley. If you watch Dubai or Basel(although not fast compare to the 90s but considered fast relative to this era), there's more attacking tennis, offensive-minded and more winners.
What's clear about a tournament like Dubai, like in 1997 when Muster won the title, is that the players are hitting flatter shots during the rallies than what they do today, because of the gut strings that were used back then. The amount of power is greater in today's game, however, despite all the topspin on the balls.
Easier to master today than in the 90s is pure subjective. Even if one can prove that an era is harder to master than the other doesn't equate to more difficult to dominate. I can argue that Roddick had it tougher than Sampras. I mean Sampras rely the most is the serve, but so do Roddick. The big difference is Sampras played in the environment that aided his serve but unfortunately for Roddick, the slow courts is to his disadvantage.
Surely you agree that the 1990s saw the greatest variation in playing styles in tennis? As a result, it was extremely hard to master all those different styles and dominate the tennis world. Sampras was the most successful player of the 1990s, yet he seldom made a big ripple in the clay-court scene, apart from his 1994 Italian Open triumph with Vitas Gerulaitis temporarily coaching him.
What was bad about tennis in the 1990s from a business standpoint, is that all this variation didn't always make for very prominent long-lasting rivalries, that could be constantly marketed, like Borg-Connors-McEnroe-Lendl-Becker-Edberg-Wilander sort of rivalries, or like Federer-Nadal-Djokovic-Murray today. In the 1990s, there were also no compulsory tournaments, unlike today. This is why I think the business brains in tennis like today's era more, because it's made prominent rivalries. Whether it's better for tennis as a sport, is another matter.
because you seem to buy Agassi's opinion though Nadal has 3 less majors than Fed? 
