Has anyone experienced a weak backend motion of Dark Legend when the lane starts to break down? Does this happen because the ball is asymmetric? I am loosing my confidence in this ball....

You'll definitely need to jump into the oil as the Dark Legend is typically best on Medium-Heavy patterns.  The other thing to remember is that an asymmetrical core will have greater flare potential.  Combine that with the cover found on that ball and that will help explain what you're seeing.

I would suggest adjusting the surface and/or changing your hand release with jumping in the oil to start. That should allow you to see some better results.

Ok. Thank you all for the insights. I have another question. If I sand this ball, will it hook more or be more appropriate for heavy oil? Because I am trying to buy another ball to add on my arsenal that can cover med-heavy oil.