I have three FNS: an FNS-40 (for which I also got a 9mm conversion barrel), and FNS-40L, and an FNS-9c. I got all three in trade, and the FNS-40L came with some gunsmithing done, and I like it best. The FNS-40 is similar in size to the Glock 19, and the FNS-40L (same frame) has a slide similar in lenght to the Glock 17. I'm less enthused about the FNS-9c, but most members of the FN Forum love it!
All three were returned for an optional* factory recall (at FN- and given a new striker design. They were out and back in a little more than a week and a half! The FN-509 shares the same striker assembly -- it is a slightly revised version of the FNS.
*With the original striker design, it was possible to have an accidental discharge the slide if was pressed against a hard surface (causing it to move a little) then returned to the starting position. That movement apparently overrode the the striker safety. There's more info available on the FN-America website and a YouTube video from the people who first found the problem.
Except for the YouTube story, I've never heard of a accidental discharge from a gun using the original striker. But...
A few of the newly redesigned strikers have failed, but we don't know yet whether this was a quirk of rushing a new part into production, or whether the new design (MIM) has a quirk. Many folks who have gotten the upgraded striker have NOT had problems. But some FNS and FN-509 owners have had striker failures. (All repaired quickly and returned to the owners at FN-America expense.) FN is considered one of the outstanding arms makers, and this sort of problem is very unusual.
APEX has introduced a new machined, stainless striker, but they are so new we've not seen any reports on how they are doing or whether they'll have their own quirks. (Apex just started shipping in the last few days.)
A lot of folks like the FNS line -- I'm one of them -- but the FNX-45 is a very popular gun, too. Unlike most polymer-framed guns, they actually feel like a real metal-framed gun in your hand.
Given the unknowns associated with the new striker (either original, replacement, or Apex-design), I think I'd give some thought to the FNX-45 or FNX-45T (Tactical), or just wait a while before getting an FNS or FN-509.