Stability hasn't been an issue for us. Virt-v2v uses ntfs-3g intensively and we have migrated a large fraction of a million VMs, many (the Windows ones) all using ntfs-3g. We've not had problems with ntfs-3g at all.

I'm still interested to see if an in-kernel driver can be better.

(Disclaimer: I have contributed a few patches to ntfs-3g upstream)