A tetrahedron composed of steel beams is very strong to shear forces, where a cube would easily collapse without other supports. On the other hand, when it comes to compression forces A cube will have four beams supporting each face and a tetrahedron would only have three.
Even then, when a compression force is applied to a face, each steel beam in of itself is stronger to forces parallel to the the length of the beam versus perpendicular to its length. A tetrahedron's supporting beams would be diagonal to the pressure and contrast with a cube's 4 support beams in parallel with the direction of force.
Now when dealing with solids, and assuming the same random non crystalline internal structure between a tetrahedron and a cube, a tetrahedron sitting with one face on the ground will have one point facing up. The the tetrahedron highest point will be comparatively weak to oppose a compression force acting on it, and contrast with the same force applied to a much larger area of face of a cube. Both tetrahedron and cube would be about the same if the force is applied to it were applied to only a little point.