Nessus is a widely-used, well-documented and open source program. These are all characteristics a security specialists appreciate. You can use Nessus to determine KNOWN security breaches in applications and routers. It does not account for social engineering, access via VPN tunnels etc. Furthermore you only get a picture of the current machines on the network. Insecure laptops may be on and off the network as well as insecure wireless devices.
So the answer is No. Nessus is not the answer to all our security problems but it is a great tool.