One of the biggest challenges will be drivers for the actual hardware - at the moment it looks like they have support for SPI and for built in LEDs. But it's a big challenge to expose all the different peripherals that come with MCUs in a consistent way.
Actually, looks like they've got quite a lot of stuff done already:
https://microsoft.github.io/devicescript/api/clients https://microsoft.github.io/devicescript/api/servers