I can offer assistance to Wallace over his/her Joy-Con rail button (SL/SR) problems.
I had never knowingly come across it in general forum browsing, etc. but just last week I encountered it on one of my right Joy-Cons, a launch day (March 2017) neon acquisition. I dont usually play with a single Joy-Con but was preparing for a get together and, like Wallace, found that the SL and SR buttons, plus the Sync button, were not producing any response when pressed.
Unlike Wallace though, I didnt find it difficult to research. I web-searched Switch not detecting SL SR buttons and the first video result was a good un.
In short, the ribbon cable responsible for powering the rail buttons inside the affected Joy-Cons has been poorly installed/designed, causing a severe kink to form in the ribbon, preventing a signal travelling through it. When I opened my Joy-Con up I found exactly such a kink in the ribbon cable. I carefully tried to see if it could be relieved enough to restore function, but the ribbon had seemingly become so brittle that it immediately snapped in two at the point of the kink.
Replacement ribbon cables can be found online – I ordered one from overseas so its not arrived yet for me to test out. A tri-wing screwdriver is required to open the Joy-Con.
In July 2017 I bought the ARMS yellow Joy-Cons, which Ive used almost exclusively ever since and the neons have just sat on a shelf, so Im in no doubt that its a design/manufacturing issue rather than user maltreatment. Not that Im taking a dig at Nintendo though – I love the Joy-Cons and accept that such things can happen.
My yellow Joy-Cons, which have seen by far the majority of my 600 hours+ Switch use (playing mostly thumbstick intense games like FRead More – Source