We have been running Nexus 9.1.0 without any issues on OS X Server version 10.5.6 since it was released. It has performed really well for us no major bugs or issues that we know of.
The only issue I know of is the lack of appletalk printers in leopard. This is a mac thing not a nexus thing. I would imagine if you are using PDF that you are not printing to workflows anyway. Besides that, everything works well here.
