Look at this movie. It shows the default 3rd person character controller, walking on a see-saw created as a simple static mesh with collision, and a rotational joint that only allows rotation around one axis.
Unfortunately, the physics is very unstable, as you can see from the motion blur happening when things shudder, and from the camera being jerked around.
Additionally, you can see the plank being twisted sideways, around an axis that is supposed to be locked.
[Video of unstable physics rotational joint][1]
Here is how the object and joint is set up:
