Why wouldn't a 15 ga finish nailer work?
Few reasons .15g nails are not made for exposure to weather.They are to small of a shank for any holding power and the heads are to small to
keep the boards from twisting and cupping .The boards will move no matter what kind of nails used but they will pull right off of a 15 g .nail.They call them trim nails for a reason.
I could go on but that should be enough.
Going to edit this: Do your own test.Take a one X and your 15g nailer and shoot aa couple on the bench into a 2 x 4.I bet you can pull them apart easily by hand. Now drive a couple ring shanks into the same wood and pull them apart.Not very scientific but you will get the idea.
Wood,especially what they sell for fences these days has a high moisture content.When it acclimates and dries out it does all sorts of weird thing like twist,cup and warp.A 15 gauge or any nail will not stop some of this from happening but a 15 g finish nail does not stand a chance.This is just the simplest way of stating it I could think of.
They also have almost no head and the wood will pull away from them in a heart beat.