One seemingly repeating weakness of Jin Yong's stories is that he creates villains that are...underwhelming. Somehow, a number of his villains are deficient in various ways that make them less of a threat to the protagonists than they should be. Cases in point:
1. Mo Yung F'uk (DGSD): ostensibly, the main villain of DGSD (if that story ever had one), but...never could quite put it together. All the heroes had much more powerful martial arts than he did, and his schemes were half-baked at best.
2. Yeung Hong (LOCH): very weak martial arts skills and not a great schemer, especially considering that he was usually opposing Wong Yung on the other end.
3. Golden Wheel Monk (ROCH): Greats-level martial arts, but had an incredible penchant for snatching defeat out of the jaws of victory.
These are the three that come most prominently to mind, but considering they're all fairly high-profile villains in the Jin Yong canon, it does seem that Jin Yong produces a number of duds among his antagonists.