There's a chance any gun you get will have issues. If you inspect the gun before buying it, you can avoid the 'issues' with the FNS and FNX for that matter. In regards to the heat 'issue' I don't really consider it an issue at all.
I personally like hammer fired guns and the SA trigger is better than the FNS's trigger and the FNX allows you to have it cocked and locked or DA/SA like a SIG P226.
My first gun was FNP, similar function, and worked for me. If you think you'd prefer the same trigger pull every time (and no decocking) go with the FNS. If you don't care and want a lighter trigger the majority of the time with the potential safety of a long heavy trigger, go with the FNX.