Underrated ...
I'm gonna say Cowens vis-a-vis Silas. The reason for it is that when Silas was traded, Cowens quit and basically never returned (at heart). Thus, he was a considered *mediocre* during the last half of the 70s before his departure in '80. As a duo, this was the heart and soul of the 70s championship drive and it's seldom recognized by others.
Then for individual players ... Maxwell, Parish, Scott. These men were serious amazing players with limited recognition.
Overrated ...
Yes, Bob Cousy, and the reason for that is that no one's ever raved about his defenses, only his Houdini tricks on the hard wood floor. I'd take JoJo White over him since in a tough series, defense wins championships. That globetrotter stuff is nice for the regular season.
Then for individual players ... Walton (common, he also had Sichting w/ him that year), Ford, Walker (but that wasn't his fault, it was the media).
Rated Correctly ...
As for Bird, his only problem was that his brain wasn't inside the body of a Dwight Howard. Then, he'd be the defensive and offense monster of the 80s, basically snagging 20 rebounds, blocking 10 shots, and scoring 20 points in the post and then when the whole opposing team crowds the paint, he'll dribble out to the 3 pt line and drill a three on them, thus matching both Chamberlain and Baylor in points per game (50+ ppg). Simple put, Bird's defenses were based on the zone defense principle where despite his slow body, he could pick off steals (~2 per game) in the passing lanes and still be as effective as many quicker one-on-one defenders.