Concerning DPI and trackpad integration, Ubuntu should strive to be as Mac-like as possible, at least imo. Macs absolutely win in the trackpad arena; multitouch works like a dream, configurable gestures aplenty to achieve whatever you want (of course, it could always be more customizable).

DPI scaling between monitors work exactly as you'd expect. Windows stay the same size when moving between high-DPI and regular monitors.

These two problems are two of the biggest reasons I don't use Ubuntu (or any Linux) desktop (I use a macbook with a headless Ubuntu Server box and, when necessary, X11 forwarding over ssh).