When using pgfpages and pgfpagesuselayout{resize to} to resize a page on to a different size page, shadings do not show with xelatex.
Setting the driver to pgfsys-dvipdfm.def allows shadings to work, but (presumably) may result in other problems.
MWE:
\documentclass[letterpaper]{article}
\usepackage{tikz}
\usepackage{pgfpages}
\pgfpagesuselayout{resize to}[a4paper]
\begin{document}
\tikz{\shadedraw (0,0) rectangle (3,1);}
\end{document}
I discovered this while trying to answer this question.