Ok so I got unity up and running. For my own amusement this post is for me to keep track of what i am doing, and encase anyone else comes across this.
I tried the straight FBX material method to expose the inherent export bugs/limitations.
I used Maya 2017 as a secondary test program besides Unity 5.6.0f3. For my example scene in Houdini I created a geometry node geo1, placed a testgeometry_rubbertoy1, I set the scale to 100 and TY to 66 (it's small in unity example scene) and began my exports. /obj/geo1/testgeometry_rubbertoy1. I tested with both fbx exporters the one in the File > Export > Filmbox FBX... or through /out/ with a Filmbox FBX rop so /out/filmboxfbx1. I didn't try the script methods with these test as it's really the same code. I set the output file to $HIP/materialTestHoudini.fbx and selected the Export node to /obj/geo1.
From Houdini to Maya/Unity:
By default it will export a material link it appears for Maya with a diffuse texture but the material path is still the native HDA reference, plus the spec texture did not come across.
In Houdini HDA opdef:../..?toylowres.jpg to this in Maya opdef:/Sop/testgeometry_rubbertoy?toylowres.jpg
So I allowed editing on the rubbertoy and saved out the texture to $HIP/toylowres.jpg, and changed the link to that from the above and the color texture comes across.
Albeit, while a few textures come across this is certainly not a good material exchange accept for some really basic parameters, the phong shader was selected with houdini specular lighting model, and incandescence was map from subsurface color, this seems to be a bit of material limit. Substance definitely is a better method.
So in unity the material link does come but definitely not materials as you said.
So what we did at most studios was write a script that would at least take this materials link and do a look up and replace the material to something pre-built. But that is all ways a pain, and a studio one off.
I tried next to just export this same exact model from Maya to Unity, and it failed too. Which this should work, however... it's lunch time so i'll edit this when I get back.
...OK so I was looking into why the texture issues happens in Maya too with FBX, I found this blender post and it makes sense.
Mischa is right, FBX does not export textures and you have to import them into your Unity Scene, but if done right you can reload your FBX and do not have to manually assign them to the materials. They will find the links with the right materials.
I always have my folder setup like this: Assets/MyModels/ModelName/ FBX imported file which creates a materials folder, I add a textures folder, my 3d model also pulled the textures out of a textures folder next to the model. after I dump the textures in the textures folder I reimport the FBX and wolla! all the textures are assigned.
So this will probably work in the end too with Houdini Textures. Need to give it a go, but if I'm thinking right this will be the same with Houdini Engine.
BINGO! Take that FBX! The whole material parameters don't map, but that goes back to common mapping issues, and why substance is the bomb.