I can't see how the polarising part of a circular polariser can be any less effective than a standard linear polariser (because that's what it is). However, it might be possible that the re-circulificating* bit is somehow reducing the contrast a slightly making people think it's less effective at polarising.
(* I can't think of the right word).
You may have hit it. It could be the contrast difference.
Moma don't take my Kodachrome away!
Oops, Kodak just did!
For all practical purposes, they've taken Kodak away.