Not too many groups with their wide range of vocals.
And the harmony was spot on.
I was never much of a fan of their music, but was very appreciative of their harmony. As a guitarist and pedal steel player, I’ve always been into harmony, very deeply, as opposed to single note playing. Double and triple stops made my playing stand out. So, even though I wasn’t interested in their musical choices, I was very interested in their use of harmony!