VoLTE certainly is an 'interesting' beast and continues to occupy the minds of thousands of engineers to make it work in networks around the globe. I wonder how this monumental effort stacks up against other VoIP services such as Facetime, Skype and others that are centrally managed!? That probably costs a lot less money and they are already up and running for many years now. So far I've seen one major advantage of VoLTE over other voice over IP service on mobile devices: It can do a fallback to 2G and 3G circuit switched channels once you run out of LTE network coverage.
Recently, another advantage came to my mind: Instead of globally managed from a single country there are several independent VoLTE systems in each country. While that creates all sorts of problematic disadvantages, the advantage of such a distributed system is that there is no single point of failure and no single point of control and interception. So if somebody wants to spy on VoLTE installations they have to make the extra effort to break into each and every system independently. Not that I think that this is beyond the capabilities both in terms of knowledge and man power of some organizations but it's a lot harder to do and a lot more difficult to conceal.
Not that VoLTE is an end to end encrypted system but at least it makes it a more local problem.