A network video system can be substantially more secure than an analog CCTV system. You can, for example, keep it disconnected from both the Internet and the corporate network and use data encryption for every camera. But oftentimes there are benefits to tightly integrating the network video system with the operations of the corporate network and having access to that video from remote locations over the Internet.
There are no easy answers to what type of security you should implement. It all depends on your environment, your application, and the value of the transmissions you are sending. It is much like choosing security for a particular building. You can use a single lock or a series of locks. If the likelihood of break-ins in that neighborhood is high and the building's contents are valuable, you might decide to add bars over the windows, alarm systems, fences, or even around-the-clock guards. Before you make any decisions, you first need to do a thorough risk assessment to determine what security technology makes the most sense for your particular circumstances. With the array of technology now available, it is simply a matter of choosing the one that best fits your needs and budget.