Lingering application issues, emulation of Intel, problems running older 32-bit Intel apps (under emulation).
It's a proof of concept, the bigger issue is, will running the ARM version Windows satisfy people who want to run Windows on a Apple Silicon Mac, because Windows ARM still has it's own issues even on official Windows hardware like the Surface Pro.
I've tried it, it works, for the most part, still a lot of work to do, but interesting. So, Parallels is showing off a Technical Preview of Parallels that can run the ARM version of Windows on Apple Silicon apps.