I do have to laugh a little about how highly regarded mil spec has become in the AR world.
Technically, no civilian has a true mil spec AR unless he has a license to own one.
I'm old enough to remember when Bushmaster was considered equal to or better than a Colt (and Cold did have their share of issues in the past, which I'm scratching my head on if they're all "true" mil spec rifles), and my, how those days have changed. Interesting enough, back in the late 90's and coming into this century, no one gave a flying a flip about a staked castle nut. Now, if a AR's castle nut isn't staked, somehow it's inferior. It's not hard to stake a castle nut
Don't get me wrong, mil spec isn't a bad base guideline to go by, but 20 years ago there were only a handful of AR manufacturers, and you knew which ones to stay away from, mil spec or not.
As previously mentioned by others, some very "high end" AR's being sold in the marketplace today aren't even true mil spec.
I have no doubt that if FN is selling a AR in the civilian marketplace, it has to be a decent product.
Heck, my old late 90's Bushmaster which had thousand upon thousands of rounds through it before I sold it NEVER gave me any problems.
Buy what YOU want, shoot the living snot out of it, and don't worry about what the internet experts say
One of my most favorite 5.56 rifles isn't even carried by the US military, I know it's not mil spec, yet over 3k of steel ammo has been through it without any hiccups.
Please don't take my posts as trying to be argumentative, I'm only trying to convey that no matter what gun you buy, you run the chance that you can have problems. That said, if you buy the gun from a reputable manufacturer who stands behind their work (customer service and pride in workmanship IMO trumps "mil spec" any time of the day IMHO), you should have a reliable weapon that should give you years of service.