The fight has started last year, when NBD replaced NFS as the default protocol for root device for thin-clients in Ubuntu. But, there was a lack of scientific data about benefits of NBD.
Few weeks ago, a team from Birzeit university started to work on the topic. They have compared boot time using NFS and NBD. Results show that a thin-client booting with NBD is 44% faster than with NFS. More testing is underway.
One big issue with NBD is that it's not as robust as NFS. For example, the client doesn't reconnect if the nbd-server is terminated. The thin-client is left in a unknown state, and must be rebooted. While this is not a big issue in small scale setup, it's not an option when you have central boot server and thousands thin-clients. The team plan to work on the availability issue as well.
Stay tuned, the final battle is starting.