You might well ask why using transparency features is unsafe in XeTeX. The answer lies in the
implementation history of Ghostscript, starting as a PostScript interpreter before PDF was invented, when
it was acceptable to abort on an error, whereas with PDF, it must keep going. The transparency
operators, if abused in relation to this error recovery, can cause internal inconsistencies in
Ghostscript's state.
Thus, to be safe, we must disallow transparency with Ghostscript by default. The xdvipdfmx backend for
XeTeX calls Ghostscript under certain circumstances, notably when using PSTricks. (x)dvipdfmx itself is
not a PostScript interpreter, which is exactly what PSTricks requires.
Thus, to enable transparency, xe(la)tex-unsafe run XeTeX with arguments to the xdvipdfmx backend
(-output-driver="xdvipdfmx -i dvipdfmx-unsafe.cfg -q -E") to use dvipdfmx-unsafe.cfg, which in turn tells
Ghostscript to enable the transparency operators (-dALLOWPSTRANSPARENCY). They're disabled by default.
There have been further complications related to reading from the filesystem, but as of TL22, these have
been solved. The transparency issue cannot be solved without a different Ghostscript implementation, and
the Ghostscript developers say they have no timeline for this.
To repeat the above: when using PSTricks and transparency, we strongly recommend playing it safe and
using Lua(La)TeX instead of Xe(La)TeX.