Although completely anecdotal and not first hand evidence, I do trust the people telling me this.
Last year, a nearby PD got a sweet, sweet deal from FN to go with 509s for their duty guns. FN took their old guns on trade and charged them less than $100ea for the 509s. FN also took their old holsters on trade and basically gave then new duty holsters for the 509s. Almost no one was happy, they think that the Chief and firearms instructor may have gotten kickbacks in order for FN to claim that the 509 was their issued duty gun. Number of Departments issuing a given gun is a big deal in duty guns and advertising. They had tested several different makes and, supposedly, went with FN based solely on price, not performance or anything else.
First issue was the barrels. When they went out to qualify, every pistol was having FTExtract issues. FN ended up having to send new barrels for every gun to get them to run. Not a very good way to start out with new pistols. They ended up having to put off qualifying with the 509s for several months until the new barrels came in, were installed and the guns test fired. No one I spoke to was able to tell me the exact issue, just that all of them had to have new barrels installed and that, apparently, took care of the issue. Apparently, none of the Dept admin who knows what the issue was is talking.
Another issue is the triggers. Almost everyone at that Dept carried Glocks, most issued and a few personally owned. Again, everyone I have spoken to complains about the triggers. Stiff, squishy, no discernible reset and a few have reported that the pull weight changes with each shot, sometime light, sometimes heavier. I haven't shot one myself, but this is what they have told me.
Based on the issues they have had, most of the Officers hate the 509s. When you combine that with the fact that FN has a tendency to just up and discontinue pistols at times and to discontinue supporting any of them that have been sold, I'd be very leery about buying any FN pistol. Several years ago, I really wanted an FNP. By the time I had the spare cash, FN had discontinued and stopped supporting them. Discontinued I have no issue with, but if there are no parts or factory support, that's a hard stop for me, for something that I may use for duty or off duty use.