The FNX is a great gun, but has a few major issues if you plan on using it as anything other than a range-toy.
In short itemized listing:
Mags picky about FMJ ammo (may be resolved on FNX series) Any ammo slightly too long drags on the front of the mag and jams up everything.
Mag retention in gun is a very, very, tiny tab of metal. If this wears even slightly, mags will not stay in gun.
Mags are hard to find and typically expensive.
Trigger pins walk out (has gotten better with FNX series blocking one of the trigger pins with the frame)
DA trigger pull is VERY heavy and VERY long.
DA trigger is too damn far away. If women need to use this gun, they won't be able to reach to the DA pull.
Possible to sweep past safety and accidentally decock gun if carried in Condition 1 status.
Possible to sweep past safety and completely lock up fire control group if you "white knuckle" in a panic and ride the safety like a 1911.
Frame plastic is soft. Have had tac lights dig up frame under normal clamping, gnaw up the picatinny rail, and twist right off the rail.
Frame texture is very rough polymids. You need gloves if shooting for any extended period of time.
It's a fucking brick.
Backstrap retention, like magazine retention is flakey and weak. Many people end up gluing their preferred backstrap in place.
Impossible to detail strip gun without destroying some parts of the fire control group. Which makes absolutely no sense since FN likes to play up the longevity of the gun due to it's "replaceable frame rails".
History of ambi mag release failing
Spare parts are almost nonexistent
FN's customer service reputation has gone down the shitter.
There is an alarming number of these guns suffering catastrophic failures due to poor barrel design. Some owners only lose their guns, some lose tissue.
outside of that it is an accurate, easy to shoot pussycat of a gun with great sights and awesome capacity.
I think you should also look at the Glock 21SF or Gen 4 Glock 21