Speaking of the subject of this thread Lendl, one thing that was underrated about him was his versatility, not just in terms of his outstanding results on all surfaces, but in terms of how he could vary his playing style.
The guy won majors on hard courts while playing as an aggressive, attacking baseliner that would look to over-power opponents and occasionally chip and charge. He also won majors at Roland Garros primarily playing as a defensive, grinder who would rally all day long from the back of the court and look to outlast his opponents. Plus he nearly won majors at Wimbledon playing as an out and out serve and volleyer that would come to the net on both 1st and 2nd serves.
So he had a lot of success at the big tournaments with 3 radically different playing styles.