This has me wondering now. I use Reaper on my MacBookPro 15 Retina 2013, and even with the built in audio it works well. I have various interfaces, Steinberg UR44 and an old Tascam MkII, the Tascam works great in the MacOS partition, but I have not installed Reaper into my Win10 Bootcamp partition. Perhaps I should try that.
So Asio4All does not work, thanks for the heads up. I will give it a try with one or both of my interfaces and write back.
Cheers