FNX9 owner here. I like it a lot. However, I had to make some minor modifications and do some ammo testing before giving it my stamp of approval.
I don’t know if the 509 is different, but I think the top of the backstrap of the FNX9 (sits on top of hand) is poorly shaped (easily remedied) and the bottom of the magwell (different on the FNX vs the 509) is pointy and can dig into your hip at 4-5 o’clock IWB if against the skin and sitting. I also found that mine shoots noticably low with 124gr, 124gr +P, and 147gr while hitting pretty much to POA with most 115gr and 115gr +P ammo… I’ve noticed this on some other brand pistols too, at least for me.
That being said, I’m happy with it now and don’t plan on replacing it.