When you are working on large moodle installations, is better to have all in separate servers. Specially when you need to do load balancing of the webservers. If you are using just one webserver for moodle, is better to have moodledata in the same, and the db server serparately,
When you have a lot of users, usually moodle show saturation in web resourcces more than database resources. So, if you are growing a lot, you can plan some load balancing in web, using more than one moodleserver. And that's the reason for having moodledata separate. Because if you are having two or more nodes of moodle, they must have sharing the same moodledata.
If for some reason you choose to have moodle data in a separate server, Using NFS has been working fine for me, and that's the recomendation from documentation. It could be better and more secure to separate the comunication between servers from the Internet, Maybe using connetions from different lan cards or by a vlan. Make sure that the brandwith is adequate for enough writting and reading speed to the moodledata directory.
Hope it is useful.