UMDF gives the opportunity to write user mode driven drivers by interfacing with an abstraction layer. It cannot however implicitly sit between a driver written for kernel mode and push that kernel mode driver into user mode. In other words, you have to specifically write your driver to run in user mode in order for it to be user mode. UMDF is also written to target PnP drivers - specifically MTP based hardware (ie, MP3 players). AFAIK it doesn't allow you to interact with the PCI bus.

Its easier to think of it like libusb for Windows. There's a generic driver provided and you can write user mode code to interface with that generic driver. The generic driver in turn handles packet composition and transfer with the USB/Firewire bus. Works well with USB/Firewire devices but probably not so well with graphics cards because of the nature of graphic card evolution.