AHR and VXGI are just two different answers to the same problem. wise, they are mostly the same, but the algorithms are diferent.
Think both AHR and VXGI are too early in development to compare other stuff, but like I said before, having to different techniques cant hurt 
Same as VXGI or LPV . It doesn’t 
For now at least, it could be supported.
[EDIT]
Let me clarify a bit. What it doesn’t handle are refractions, caustics, and tracing transparent surfaces. What do I mean with that? When voxelizing, alpha masking is taken into account, but the voxels that end up filled are all considered as opaque. Also, you can have a particle system, which uses alpha blending, and get GI for it (not getting it to affect GI, getting it to be affected by it), it’s all the same as tracing runs in screen space.