This is stated in the man page for systemd-nspawn
Note that even though these security precautions are taken systemd-nspawn is not suitable for secure container setups. Many of the security features may be circumvented and are hence primarily useful to avoid accidental changes to the host system from the container. The intended use of this program is debugging and testing as well as building of packages, distributions and software involved with boot and systems management.
This very question was subsequently asked on the mailing list in 2011, but the answer seems to be outdated.
systemd-nspawn contains code to execute
CLONE_NEWNET using the
--private-network option now. This seems to cover the private
AF_UNIX namespace issue, and I guess the
CAP_NET_BIND issues mentioned.
What issues remain at this point and what does for example LXC do in addition to what
systemd-nspawn can currently do?