POD capacitors vs. Poly-Poly capacitors

Hello,

What are the drawbacks to using poly-on-diffusion capacitors instead
of poly-poly capacitors ? If pod capacitors are more capacitive than
pp capacitors, why are pp caps used ? Are pp caps better ? Is their
lineraity coefficients significantly better than the pod's ? Obviously
the pod cap diffusion node's voltage cannot drop below VSS (or rise
above VDD, if the substrate is N-type), but assuming this node's
voltage stays within the [VSS,VDD] range would there be a significant
difference if  pp caps were used? Are the pod caps more resistive than
pp caps? less reliable maybe?

-Frank