As for whether Flowers was underrated. Hes difficult for me to place. He was very very hard to outpoint but he was also very susceptible to punchers. He threw slapping punches which arent technically scoring punches and can be ruled a foul. Because of this some stricter referees docked him points etc. He was also, like Greb, wild with his punches, believing in quantity over quality, and a result he could be prone to fouling. Hes the type of guy who could give anyone hell but you wouldnt want to bet on him because he could break your heart.
I think it's styles .. Greb had a much better chin and beat many men that either did or could have beaten Flowers but H2H Flowers matched up well with Greb as Harry was no big puncher and they were both unorthodox , huge volume speed demons .. Flowers was older than Greb and also had a ton of miles on him ..
Flowers turned pro five years after Greb. By the time Flowers turned pro Greb had already had almost as many fights as Flowers would tally in his entire career. Greb had WAY more miles on him than Flowers. And as the saying goes, in boxing, its not the age its the mileage.
I see my comment on this thread when completely over some heads, 5 years ago. To spell it out : If you were a Black fighter back then it was an inevitable right of passage that you were going to eventually get knocked-out by Sam Langford. "but not Jack Dempsey" ie. if you were a Black fighter back then it was inevitable that you would NOT be knocked out by Dempsey.