Xinerama came with the price of not supporting mixed DPI. It was logically a single screen, just with irregular shape, so all screens had to be same DPI (and bit-depth, but that's not an issue nowadays).
Bit-depth is certainly an issue these days. I've got a 1920x1080 60Hz 8-bit/color 24" monitor and a 3840x2160 144Hz 10-bit/color 27" monitor. Different resolutions, refresh rates, color depths, sizes, and DPIs, on a very modern system.
