Well, it's always safer to use widely used solutions in production. That way you encounter fewer bugs, and it's much easier to find a workaround in case you do encounter one.
It's slow and doesn't run any software people usually want to run in production. If your "production" needs are something Plan 9 is actually tailored to, sure, go ahead.
I run it as my primary development environment, but what I do is pretty specific, 99.9% people need something else out of their development systems.