I have recently been told that in the FDA environment, PDA detectors are only acceptable for method development and some stability assays, unless otherwise specified and are considered to be unacceptable for assay determinations. I was surprised by this and wonder what the rationale is. Any help?

Wrong, wrong, wrong!

The origins rest in the fact that early PDAs were somewhat less sensitive than variable-wavelength detectors. That sensitivity gap has (mostly) closed.

So long as the method has been validated (i.e., you can demonstrate the required accuracy, repeatability, detection limit, . . . etc.) there are no regulatory limitations on detector choice.