One answer to that, given that the ME is 100% black box and has unlimited access to the system, is “no”.
If you have a separate dedicated NIC then you avoid certain practical attacks although I don’t see any reason why the homunculus CPU firmware couldn’t in theory contain a device driver etc. for the separate NIC.
Network access (trojan horse for remote inwards access, exfiltration) is only one part of security fail. The homunculus CPU could be used by a legitimately authorized local user to conduct a privilege escalation attack.
The security fail is limited only by your imagination.