O_DiRECT is insufficient because you have no way to coordinate with the kernel to drop io buffers when there’s memory pressure
Not saying more work in the space wouldn't be welcome and interesting, just that it's generally the assumption whenever I've seen O_DIRECT (since the Solaris days and through today) that what's using it is the only thing of substance on the box.
Besides, O_DIRECT can be used at the file level (and the file system level as well via chattr – if the file system supports it) whereas the raw devices are, well, device level only.
But yeah, I understand you weren’t talking about buffers, I was just highlighting that O_DIRECT and raw devices are totally separate problems from having a user space cache that can be evicted by the kernel memory and reconstructed by the application lazily.
I've not yet bothered to check how things work with raw devices in Windows. I believe only Admins can even operate on them so it would be kind of limited usefulness.