It is hard to compare any athlete from another era... but it is obvious they were skill tennis players. The thing they could not have taken advantage of is the technology of today, plus the players of today are true professionals... they cross-train and are at the highest levels of fitness. Back then I am sure they went and had a beer after the match.
So were the good... they were very very good. Could they have been better with todays technology and training...absolutely.