The FNAR is a fine setup if you're looking for an economical grab and go setup.
They are relatively accurate, reliable, and durable, and pricing is pretty good, but they do have some cons.
Mags are relatively expensive, but they are steel mags and they will last a very long time. If you're looking to stack them deep then it's going to get expensive. If you're okay with having 5 or so then you only need to purchase 3. Remember, this is not a battle rifle or a fighting rifle, it's a DMR setup, so kitting up with 10 loaded mags may be a little more than you need for that task.
The rifle design does not lend itself well to customization as others have accurately noted. Even threading the barrel is going to be a challenge. I agree with others that FN dropped the ball on not offering threaded barrels.
There is practically no aftermarket support. Basically, what you see is what you get.
The rifle is a challenge to disassemble and clean. Remember, it's based on the 1918 BAR design, so you know it's reliable and you don't need to clean it very often, but it's also a challenge to detail strip and clean. It can be done, you just need some patience.
For what the FNAR is, it's not a bad rifle. If you realize and accept it's limitations, it'll serve you well. If you want to have the option of upgrading or customizing a rifle, this one is probably not a good choice.
USSA-1