If we accept that the PNLF method yields accurate distances, it seems odd, at first, that the errors from population differences and extinction aren't larger. A potentially dominant population effect is age, as discussed already. The key point is that intermediate age populations all produce nearly identical central star masses. This follows from the initial-to-final mass relation (Weidemann (1987)). That is, for progenitor masses between 1 and 2 M corresponding to ages of about 1 to 10 Gyr, the central star mass will be in the narrow range of ~ 0.58 ± 0.02 M. This narrow range is close to that observed for white dwarfs (McMahon (1989)).
Another important effect arises in young (< 0.5 Gyr) populations to inhibit [OIII] luminous PN from forming. Kaler & Jacoby (1991) showed, and Dopita et al. (1997) confirmed, that for young progenitors producing central stars more massive than 0.65 M, the surface abundances are strongly altered such that nitrogen is greatly enhanced. The added nitrogen competes with oxygen in cooling the nebula, to the detriment of the [OIII] luminosity. Thus, PN deriving from young, massive progenitors fail to populate the high luminosity end of the PNLF and the effect of a young population on the PNLF is lost.
Similarly, metallicity seems like it ought to play a large role. A competition exists, though, between the efficiency of the nebula to radiate in [OIII] and the luminosity input from the central star. Higher metallicity values enhance the nebula's ability to radiate at 5007. The central star, however, is predicted to have a lower mass and luminosity as a consequence of experiencing higher mass loss prior to leaving the AGB. The reduced heating compensates to first order for the enhanced radiative efficiency as metallicity increases (Dopita et al. (1992)).