In my opinion, VoLTE was a poorly developed standard. It wasn't standardized as much as it should have been which allowed equipment manufacturers to come up with very different implementations and supported operations. This put the onus on providers to test and certify each device will work on their network with all their equipment. But the basic summary, VoLTE is not standard, so compatibility is always up in the air. For all involved this adds all kinds of extra costs and complexity. It's created a step change in device support, rapidly obsoleting devices, even relatively new devices. From the providers point of view, testing and certifying devices is costly and time consuming and they are certainly aren't going to get to all of them, especially with a lower cost provider. From the consumer standpoint it's hard to determine compatibility and obsoletes many devices, same that even have the required features. It is the marching forward of technology. While LTE has worked wonderfully for data networking, the voice features of VoLTE have stumbled. Considering that networking services have been working quite reliability for years and moved through 3G, 4G and 5G and various sub versions, this is one of the bigger standouts of something that hasn't worked well.
It is also my opinion that straying from mainstream hardware and services increases risk, decreases performance, lifespan and compatibility. It's a trade off, usually versus costs and sometimes feature sets. I personally tend to stay in the mainstream. The more people using and looking at devices and compatibility, the more attention it gets, the more likely it is to be tested and validated, the more likely it is to work and problems to get fixed.
