One of the major claims of the Halo upmixer is its downmix compatibility. To do that, it has to extract all it upmixes from what is already contained in the recording. This is my preferred approach for material I want to upmix but do not have in multitrack format for remixing any longer.
The approach of using 3D reverb to create the extra dimensions is good for adding mono or stereo sources to an Ambisonic mix. There's some good 3D convo reverbs floating around these days too. I'll have to check out Ambi Verb HD.
If you are interested in using regular stereo FX in Ambisonic configuration, I have created some track templates for this here....
https://forum.cockos.com/showthread.php?t=185676