This is not totally true. It's based on slackware 12. You can download almost any slackware 12 package, drop it on your flash, update the config/go script to do an installpkg (packagename) and an application can be added.
I have my caching DNS, tftp, pxeboot, syslog-ng, nagios, X10 control processes and other administrative functions running on it.
I also run lighthttp and rtorrent on it all day long.
It just takes a bit of tweaking to install the package and insure your configuration files are in place on the root ram filesystem.
If you are savy, you can decompress the compressed initram and install the applications there.
There are members of the forum extending unRAID's functionality all the time.
Perhaps there are applications which are just not prime time for linux which does not serve everyone's needs.
Lately people have been getting vmware running on unRAID.
This is how I've been doing my transcoding with a virtual windows machne.
So the possibility exists.
It's more based on needs, what third party apps are desired, knowledge base and desire.
It's write performance would be a show stopper for any high throughput situation. The cache drive functionality does help this.