Check this doc from Physx regarding constraints:
http://docs.nvidia.com/gameworks/content/gameworkslibrary/physx/guide/Manual/Joints.html#configuring-joints-for-best-behavior
on to of that, take into account that UE4 scales drive forces by mass of the constrained objects.
Some people managed to make stable setups but it takes a lot of tweaking and experimenting just to make a single rig work in acceptable way. Physx folks recommend to use articulations http://docs.nvidia.com/gameworks/content/gameworkslibrary/physx/guide/Manual/Articulations.html for where joint stability is highly desirable, currently its available in physx but not exposed in UE4.