Complaints against the FNS mainly involve the trigger (which isn't horrible no matter what anyone will tell you) having too stiff a pull (most come in between 5 and 6 pounds with a consistent pull) though being intended as a service gun, is acceptable. A hair trigger on something a LEO might be pointing at someone is never a good idea.
Complaints against the FNX are that it has a higher bore axis (not much but noticeable) so it has more muzzle flip than it's striker-fired brother. Since hammer-fired means a bulkier slide and more weight at the back, this is expected though it's still a sweet shooting firearm. Also, a great many have complained about the placement of the safety, so make sure you actually not only get hands-on with one before you purchase, but also get range time to see if you're one of the guys who takes a safety to the back of the thumb.