I mean it's a royal bitch to get different DPI on a screen by screen basis, and impossible to do so in a standardized, consistent way on a window by window basis.
The problem with the state-of-the-art X11 is it (Xinerama or RandR) implements multi-monitor as a single logical monitor with each output stitched together. Getting multiple DPIs on each monitor is effectively impossible because you only actually have the one logical screen.
I believe it's possible with the older X multihead method, Zaphod mode, to have fully separate X11 screens, which could have different DPIs on each screen. The problem is there's no way to move windows from one screen to the next, and my understanding is that is an architectural limitation of X.
By that, do you mean it makes DPI assumptions that you'd want to break with a higher-resolution display?