Well I am gonna reply to my own thread in case someone else comes across similar problems.
Mass scale does work… but only if you have no entry for mass in the BP and the engine is using its pre-computed mass presumably derived from the mesh volume. (and possibly the density from the relevant physics material?)
If Mass scale is used the strength of the material is altered in the same proportions.
The net result is that I still have the same problem. I cannot make an object strong enough without giving it a totally unrealistic mass
Whether I use density in the physics material BP or use mass in the pawn BP I cannot seem to alter the strength to mass ratio.
Maybe I will try altering the gravity for the specific objects ?
Yes, mass scale only works if you have not overridden the mass by ticking the mass option in the physics setting and filling it with your own value.
If you have done that, then assigning a physical material and changing the mass by its density seems to the only way to alter the mass during runtime.