The one thing that these small caliber, high velocity weapons seem to have in common across the board is a big muzzle flash and lots of noise.
Im not discounting the effectiveness of the round, but if you cant see, or hear after the first shot, especially considering the defectiveness of the smaller bullet is dependent on multiple hits.
What good is it when you cant see, or are disoriented by muzzle blast to the point of slowing down your follow up shots. The .45 and 9mm have been doing a decent job all these years without all the flash and noise, and the platforms these are delivered in are usually less expensive, and easier to come by.
To me, the FN 5.7 is kinda like a Ferarri, its flashy, expensive and goes really fast, but its not going to bring home the groceries as comfortably as an old pick up truck.
If flash and blast are disqualifying characteristics, then .357s are no good either. A place for everything, and everything in its place, I don't think this was intended as a replacement for a .45 or 9mm. I have an ageing friend who wants a semi auto pistol but due to reduced hand strength has problems racking most slides and controling recoil, he shot this gun recently and is now shopping for one. I have read of people using them for short range varmint control, but beyond what a 45 or 9mm would be good for.