Not quite true. 3T drives DO work with unRAID. But you have to follow special procedures to configure them to be only 2.2T. When unRAID is enhanced to support 3T drives, you will be able to restore the drives to their full size.
With the price of these drives falling to under $170 US (that is $113 dollar adjusted for a 2T drive size), and knowing that it will make your remaining disk slots go 50% further, it might be worth considering.
So, for example, if you only have 3 more slots before you have to start swapping out your smaller disks, you'd be able to grow by 9T vs 6T with 2T drives.
And when you start replacing smaller disks with larger ones, your money doesn't go nearly as far. Upgrading 3 1T drives to 3 2T drives (which gives you 3T extra space) would cost you $210 (with very cheap $70 2T drives)
See THIS THREAD
Update, since this was written the sale price of 3T drive has falled to $129 from Newegg and $109 from Microcenter.