As far as I can remember its multiplatform - definitely Windows, and presumably Linux (as its built on Linux) - I don't remember any mention on the forums of it not working on Macs.
Im not overly familiar with FreeNAS, but I think UnRaid is unique in its use of a single parity disk for data protection (whereas FreeNAS uses standard RAID options (0,1,5 etc). For example RAID 1 requires a 50/50 split of data and parity drives, and even RAID 5 require 1/3 of the disks for redundancy (don't quote me on that figure). I believe all discs have to be the same size too.
UnRaid on the other hand uses a single disk for parity, so if you have 10 disks, 9 of them will be data disks. What's more, all the disks can be different sizes (as long as the parity disk is the same size or larger than the largest data disk). Which is great as you can use whatever old disks you have lying around.
Also with the more traditional RAID systems I believe, if you have two drives go bad you lose the entire system, total data loss. In UnRaid, you have protection for one disks loss (the parity disk will rebuild it when you replace the busted drive with a new one), but if you are unlucky enough to have two drives go at once, you only lose the data on those two disks. So in a 10 drive system for example, if you lose two data disks, you still retain valid whole data on the remaining 7 disks, rather than losing the whole lot.
Like FreeNAS, UnRaid is fairly open source, you can add to it with additional code where you want to. Don't be put off by that either, I had never touched Linux before using it, I just copy and pasted code from the Forum or Wikipedia guide for the functions I wanted to add. For example I wanted 'Wake On LAN', so that the server can go into sleep mode when not in use, but I can send a 'Wake on Lan, packet from my iPhone over the network to wake the server up again when I want to use my Sonos system or whatever. You can also set it up on a timed start, so that it comes out of sleep mode at a set time, eg 5pm ready for you arriving home when you might want it ready for use - that's what I am going to have a go at implementing next.
There are many many more things you can get it to do.
You're best off checking out the UnRaid website (Lime Technology is the company name - though I think the company is just one or two guys), which also houses the very friendly and useful forum, and also the extensive UnRaid articles and documentation on Wikipedia.