my prolem with Unity Humanoid joint articulation

I made these boxy style robots like 80's Transformers, Gobots, Gundam etc and im having trouble working out how to get Unity to work correctly as Humanoid Rigged characters, does anyone know if its possible to rig up mechanical joints that the Unity Humanoid IK system wont just ignore?

a little video clip showing the rotation constraints i'd like to have on the Humanoid rig
https://youtu.be/Eo9sV-NJ2hw

so far my attempts to use Humanoid avatar with this has resulted in the IK doing weird things, mainly joints rotating in ways i want to restrict, but ive also had instances where meshes that i had skinned to the aim constrained shoulder joint are re-targeted to other joints on the arm. i dont understand Unity's system enough to figure out whats happening

a little clip showing some of my attempts with the Humanoid rig system
https://youtu.be/1iLH7UX3YMM

has anyone else managed to get mechanical results with Unity's Humanoid rig system? any suggestions? or should i just make do with a Generic rig?