Network appliances are well suited to the task of serving media to multiple workstations, but they aren't necessarily great backup appliances. Media files are generally large and the required data rate for streaming media is relatively low. Consider a 1-hour, 1GB HD movie file. Streaming 1GB over the course of an hour requires only 0.27MB/s. That's a fairly easy task, even over a weak wireless network. If you want to back up 100GB in an hour, and that 100GB is comprised of a million smaller files, that's when you need some more muscle behind the file server.
Performance of network storage appliances varies greatly
Network file sharing is a CPU-intensive task, so targeting an actual Mac or PC hosting the network sharepoint will likely offer a significant performance advantage over cheaper network appliances. If you choose to host your storage on a network router anyway, caveat emptor. It's difficult, or impossible, to determine how a network appliance will perform based on its specifications. Vendors of network appliances rarely report CPU specifications, choosing instead to report performance in terms of achievable bandwidth. The actual bandwidth that you achieve, however, will be based on the number of files you're copying, the file size distribution, and the number and size of extended attributes in the source data set. Copying large files (e.g. media files) to a network volume will achieve the maximum potential bandwidth, while copying lots of small files will take quite a bit longer due to network filesystem overhead.
If the data that you're backing up consists primarily of large files, e.g. music, photos, video — backing up directly to a network appliance will be fine.
If you're backing up system files or applications, or many files that are smaller than a few MB, we strongly recommend that you back up to a disk image on your network appliance to improve performance and to maintain important filesystem metadata.