Samba TNG is still in alpha stage and considered not suitable for production use.
From Luke Leighton's original rpcclient man page: "WARNING! The MSRPC over SMB code has been developed from examining Network traces. No documentation is available from the original creators (Microsoft) on how MSRPC over SMB works, or how the individual MSRPC services work. Microsoft's implementation of these services has been demonstrated (and reported) to be... a bit flakey in places.
The development of Samba's implementation is also a bit rough, and as more of the services are understood, it can even result in versions of smbd(8) and rpcclient that are incompatible for some commands or services. Additionally, the developers are sending reports to Microsoft, and problems found or reported to Microsoft are fixed in Service Packs, which may result in incompatabilities."