I would agree that the downside to the FNC are parts.
There is only one reliable source for FNC parts and that is Chris at Vulcan Imports. However, Chris is unable to get bolts, bolt carriers, or barrels.
The only reliable source of these three parts is semi hosts.
These three key wear components are virtually non-existant as "parts only" and as a result I don't shoot my FNC much at all. Since I have owned it I beleive I have shot my FNC exactly twice.
I probably shouldn't mention this (as I don't need the competition) but I have also had a script that runs on the auction sites every day looking for spare FNC bolts, carriers, and/or uppers and over the past year I have found exactly zero for sale.
I have actually began monitoring semi FNC sales, looking for a "decent" deal to pick up a spare semi gun to canabilize for parts when that eventual day comes.
Unless you really want an FNC to round out an NFA collection and to use as an occasional shooter, I would suggest you look at an M16 for a few $$$ more or the AC556 if you want a more inexpensive centerfire rifle caliber machinegun. While particular AC specific parts don't grow on trees, Ruger can still provide service on these guns, post sample parts kits are available, and the major wear components, barrel, bolt, extractors, etc. can be havested from a $400 mini-14 should you get desperate.
Just my 2 cents
James
Austin, Texas