If the bullet is too soft it can upset too much, filling the chamber throat or a revolver forcing cone to a degree that can actually raise pressure over what a harder bullet would give you with the same load. It also means the bullet is overly distorted and that tends to be bad for accuracy. Also, softer lead shaves off a soft easily, so bore roughness is more prone to cause leading with one if it is fired at too high a pressure.
I did some experiments with a Ruger Super Blackhawk .44 Magnum and very soft (almost pure lead) cast Lyman 429421 (Keith type bullet), in the early '70's. I discovered that the bullets were loosing their shape at moderate velocities and rifling marks were observed on the nose of the recovered bullets where no rifling marks should have been...there were beginning to look like wad-cutters as the velocity increase, bringing about concerns about possible high pressure.