I wanted a 2016 fxs and had a chance to get1 at a pretty good price but decided to go ahead and get the 2017 fxs because of improvements like new motor, wider belt, single long brick battery option and supposedly fixed the over heating issue the 2016 had. Now the 2018 has even better improvements.
All that said I love my 2017 fxs not that much difference from the fx, tires and suspension set up a little different maybe a few other minor things, I have just under 5000 miles on mine I ride it a lot it's the 6.5 single brick. I had 1 problem had onboard charger failure it was replaced at no cost they even picked the bike up and delivered it when done no charge for anything and I live 1 1/2 hours from my dealer Letco Cycles those guys are awsome
This was really a minor thing but when the bike was new it came from zero with way to much belt tension, I rode it around and it was just making a lot of noise checked belt and way to tight I had some odd marks on the belt so called my dealer they offered to bring it in and replace it or ship me a new belt I had them ship me new 1 at no cost. saved it for a extra and adjusted the original belt to the new specs and bikes been quiet ever sense and still using the original belt, new 1 still hanging on the wall.
A second very minor thing was the terrible nasty metal grinding sound the rear brake made, as mentioned in a post on this site I replaced it with EBC Organic pads $20 noise is gone and these felt much better over oem to me.
The belt and rear brake pads taken care of when bike had under 40 miles on it early 2017, the failed charger was early march 2018.
I have had 0 other issues , love the bike, put new tires on it not long ago but ride it so much gonna need a new pair next year. I'm 6'4 220 my daily trips are part town 40-45 mph but with mostly 50 and 55 mph around 43 miles 22-26% battery left.