A neck can be too flexible, having a low enough frequency to kill response. Stiffer necks can accentuate higher frequencies over low frequencies which might sound less warm. There's a range of neck frequency that will boost a low frequency range, and attenuate some higher frequencies so that the response might be more balanced and flattering. More cross section (thicker neck) and flat sawn Maple will be stiffer than a thinner neck made of softer Maple or Mahogany. And yes, the fretboard is part of the cross section, and Ebony is stiffer than Rosewood.