I absolutely love flying prone. I like the view, I'm able to better see what the wind is doing in trees while scratching on a ridge, and I believe I have overall better view where it most matters.... above, below, and to the sides. But I do neck exercises as well to keep my neck flexible and strong.
Also, I don't agree with the arguments that supine/suprone is safer than prone. When on approach, one is usually in an upright position more or less, no matter the harness style, so all harnesses are pretty much equal in terms of safety, imo. Perhaps scratching could be considered slightly more dangerous while prone, but being prone offers less drag (yes, it's proven), therefore prone is likely safer if scratching in the same conservative fashion. I'm sure there are others that will vehemently disagree. I'm bettin' they don't XC.
Surely, having the supine/suprone options will enable older pilots to fly longer, which is a big plus.
One older, very good pilot that I know of is flying supine and he is a very strong proponent for it and argues rather strongly (quite irritating at times) about it's advantages..... but I still don't buy into it. I've never seen him do any XC flights.
Another old crow (an awesome pilot) with painful knees is considering changing to supine. Not sure why he believes it might be easier on his knees. Perhaps his neck hurts too. He continues to strongly advocate for it, but I don't see any advance from him in that direction.... he's always flying prone and kicking everyone's arse. I don't think he'll change until he absolutely has to because deep down inside, even though he denies it, he knows that he will have a lower L/D and higher sink rate. And he does a lot of XC flights.
What I know is that any extra drag absolutely sucks, unless you're perfectly happy boating about in ridge lift all the time.
Solution: Daily resistive neck exercises. This is a sport, so I train for it. Otherwise get yourself an upright harness.