Hi Derek,
I haven't tried Redshift in Cinema 4d so I can't tell if the limitations we experience have to do with Redshift itself or with how the program was implemented in Archicad. But I share your struggles with transparent materials. Yes, transparency works way better than alpha. Another issue is that ambient occlusion seems to be equally strong for transparent materials as for opaque ones. I've made a wish for an option to exclude ambient occlusion for transparent materials in this post: Fix bugs in Redshift renderer - Graphisoft Community
Next thing I noticed is connected to the weird fact that you need to uncheck "sky visibility" under environment settings to get light from an HDRI to the color channel. However, unchecking sky visibility makes you lose sky refraction in transparent materials. So in order to get a complete representation of transparent materials you need to render two pictures and add them together in e.g. Photoshop. I've made some test renders to illustrate what I mean. Notice the lack of sky refraction in the bottom of the glass spheres in the first picture. In the second picture only the refraction is rendered. Lastly I added the two in Photoshop. It is also worth noticing that the checkbox "Sky Refraction" seems to have no effect in Redshift. //Karl
