I have not yet made any conclusion, but truth is that shortly after having bought the M I started to always carry a 12% grey card to set wb. With the M9 I rarely had the need to set wb manually.
On top of that, I also noticed that the colors generated by the Adobe "standard profile" for M dng's are "generally" inferior to ooc jpegs, which makes accurate wb setting even more critical. Also my Xrite Passport generated profile returns colors which do not usually match ooc jpegs.
Even though I did not make any "scientific" study on the above, I believe that Adobe's "standard profile" and wb tables for the M need some more work.
My workaround is to always shoot dng + jpeg fine, and select after the fact based on exposure and wb.
Other than my personal issue with M's wb (auto and presets), and the quite irritating startup delay (firmware fixable, I hope), right now there is very little that would possibly push me back to the M9.