I run a pair of 27" qhd monitors at work - so functionally identical in terms of width/height to one of the new 49" LG 5120x1440 monitors.
Even that wide I have zero neck issues using them all day
Naturally we make horizontal micro head movements all the time (think about what your head does moving around all day walking/running) - it's craning your head back that gives neck issues (same issues we get climbing - 'belayers neck')