For a while now I’ve had a problem using the 3DS Max biped inside of Unity. Specifically twist links and how they would always react badly when imported into the engine. While the rest of the rig would animate like normal, the twist links themselves would fly all over the place. The only solution I could ever find was to just not use twist links at all for my Unity projects.
Recently I found out how to fix this issue. If you rig your character with twist links and then go into Max’s Schematic View (up by the curve editor) you will find that the twist links, as they appear in the tree diagram, are not linked correctly. Somehow they are able to animate perfectly fine inside of Max, but once they are imported into Unity, the FBX reader looks at the links from the schematic view and animates the bones accordingly (which makes the twist bones fly all over). All you have to do is reorder the links in the schematic view. The hand links to the forearm twist which links to the forearm which links to the upper arm which links to the upper arm twist which links to the shoulder. After this is reordered, the animation continues to play fine in Max but now also works perfectly fine inside of Unity.
I found this info really exciting! I’m happy that I can start using twist links again because it was a real pain to work without them. Hopefully this can help someone that may be running into the same issue.