NDAS is more of a solution for home usage and I didn't experience any problems during the time I used it. It's not a solution for everyone and I don't think i suggested this. As I was only sent one enclosure and only had one spare hard drive I wasn't able to test the RAID ability or how well it works, but I would think it would increase the traffic on your network as it would have to copy the data to both drives at once.
It might be worth having a look at Ximeta's site for more info, although there isn't a whole lot there - http://www.ximeta.com/technology/ndas/technologie1_en.php
And of course there would be a performance degradation with multiple users using a single drive, this is always the case, try sharing a hard drive in your PC over a network and have multiple users accessing it...