It’s the only dynamic GI system that supports every type of light, that offers multibounce almost for free, and that has specular tracing as well. However, Epic is not going to integrate it directly, because they want to be completely vendor agnostic when it comes to integrating tech, and their target system spec is currently consoles, where tech wouldn’t be able to run at all because they aren’t fast enough. If you want to use it, your best bet is to grab it from /tree/VXGI-4.9 (or https://github/apanteleev/UnrealEngine/tree/VXGI-4.9 for a slightly more up to date, experimental build).