The fact that it's all self-contained and usable on a network which doesn't have useful outside connectivity makes Nessus very useful in some environments we have in our estate. Being able to be in complete control of every aspect of how it runs is great. Also being able to size the underlying host and its network architecture to be able to scan what needs to be.
The GUI, while a little intimidating, and initially a bit confusing when you start to drill down to individual aspects, is fairly straightforward and shows you what you need. Since the refresh/upgrade a few months ago it's also somewhat more responsive, which is good. It's always good to know that as it's cloud-based the plugins are always the most recent version, as are any fixes to any aspect of it. I am a little concerned that repeated scans tend to cause some findings to appear and disappear, but that is something I'm taking up with Tenable Support at the moment, who seem to be fairly responsive, if a little slow to take RFEs on board.
We can quickly scan our network and identify any vulnerabilities based on various industry standards such as NIST, NVD CVSS, etc. We can quickly see a detailed report prioritized by severity. The included remediation tips are helpful in quickly resolving the issues identified in the scan.