Pen tile displays use an uneven sub-pixel count (2 red, 2 green, 1 blue), the actual rendition of content is dynamically adjusted for in the display driver. This works fine for irregular patterns (like images, movies, most games etc.), because this trickery is not detectable by the eye (as long as the pixel size is small enough based on the viewing distance).
The problem is font rendition and the technology OSs use to perform font smoothing (anti-aliasing and sub pixel rendering), as all OSs expect to deal with panels using equal RGB sub-pixel counts. That means they perform font smoothing computations based on wrong assumptions. This is made worse by font smoothing on non-Apple OSs being less efficient anyhow (the Quartz engine in OS X and iOS scales font width, height and weight linearly, other OSs only scale width and height linearly - this results in the actual font weight being different/off at different magnifications).
If you market a 400-something ppi display, people expect to get a text rendering quality that is identical to an Apple "Retina" device. Due to the combined effects of the PenTile sub-pixel layout and less effective font scaling by the OS, they simply will not get that. It is a clear case of designing for an impressive spec sheet, but not for the best results.