I went through a similar phase trying to use an OpenWRT router plus external hard drives for file sharing. The main problems are performance and reliability. I can only get 20 MB/s read and 10 MB/s write using a decent router with a decent processor (MIPS 74Kc). Plus, USB connection is much less reliable and sometimes the drives would get disconnected.

From the kickstarter page, Plug can only do 30 Mb/s, which is ~4 MB/s and likely not enough to stream Bluray ISO. A fast enough processor is essential for NAS.