The biggest problem is that I have multiple monitors with mixed UHD and HD resolutions - including 192DPI, 144DPI and 96DPI monitors - and the start menu is not sized correctly when using the start button on the individual monitors.
Windows 10 adds a slider (per monitor) that allows the UI to be scaled via the system - which makes it easier to test an implementation with non UHD monitors (install two 1600x1200 monitors - and set one to 200% - only in Windows 10).
Here is the link to look at about get the Start Menu implemented with Per-Monitor-DPI awareness.
https://msdn.microsoft.com/en-us/library/windows/desktop/dn469266(v=vs.85).aspx