Penhold certainly gives an advantage to the forehand, but we are now living in an era where the best penholder in the world is backhand-dominant and lacks a strong forehand. Xu Xin's style of playing big swinging FH power-loops from behind the table was good in the celluloid ball era, but he himself has said that the modern 40+ ball puts his style at a disadvantage.
RPB has a lot of advantages in the modern game -- it's compact and deceptive, and it's naturally a looping stroke that comes with sidespin. While having a strong forehand is always a good thing, Felix has shown that it's not a necessity in the modern game. One can make up for a weak forehand by having good aggressive RPB.
Even when Ryu was in his prime, his lack of backhand was a noticeable weakness. He made up for a lot of it with good footwork, but even still it was not enough. Let's be real here -- Ryu was never better than Wang Hao. His win at the olympics was mostly due to Wang fumbling the mental game. Ryu was an incredible player, for sure, but his inability to do anything besides block/punch with the backhand prevented him from being one of the greatest players of all time.
A good contrast to Ryu would be someone like Ma Lin -- Ma played mostly one-sided as well, but he had much more creativity and finesse with his TPB. If Ryu had made an effort to play more like Ma, I think he could have been the best in the world. But alas, he only used his backhand as a placeholder so that he could get in position to do a forehand drive.