Advantages/Disadvantages of high/low transconductance efficiency (gm/Id) of NMOS/PMOS
Higher gm allows higher bandwidth, and lower thermal/random noise.

Lower gm allows smaller (less area needed) for active loads or mirrors in transconductance amplifiers. I've designed 50 nanoAmp per volt "opamps" used for simple filters in detecting lock in PLLs. Low gm was useful; I achieved that by using extreme L/W ratios. Lower gm also provides higher Von at the same IDD, thus better device matching should result.