Comparing the stability of stacked spheres and boxes with physics from A-Frame Extras (Cannon.js). The boxes appear to bounce off each other in a feedback loop until they fall over, while the spheres are surprisingly stable. This problem occurs wherever a box makes contact with another box (unless one of the boxes is a static-body). Lowering the force of gravity helps as does using pre-generated box entities.
The same issue can be seen in some of the official Cannon.js demos (select the "convex on convex" option).