Our beloved repros ARE made from much softer steel then most modern guns. Shrugs.. Its what it IS.
That being said, the "soft" steel of our modern guns is STILL better then the iron, pot metal, and even real steel of yesteryear.
You will NOT blow ya hand off using hard cast balls... But what you MAY, and probably WILL do, is to wear out the rifling and forcing cone much faster then it was designed to be.
If you hard rounds once or twice a year you will get wear.. If you shoot 100 of them a day you will get VERY good at using BP... But you will also need to replace your handgun within a year or so... They ARE cheaper then most, but that dont make em FREE...
So, it pretty much depends on YOUR situation.. Do you shoot a LOT? If not then why would you care either way, go with the soft stuff! Do you shoot a very little? Well then again, why bother with hard stuff? Pay the extra for what its DESIGNED to use and youll save in the end...
If youre looking for DEFENSE out of a little 31.. well.. I would look to the 44 myself, but know that in ANY caliber the soft stuff has a "SPLAT" effect that the hard stuff doesnt... That being said, it DOESNT have the penetration that hard casts do...
So.. I suppose.. All things considered, I cant really SAY without knowing what ELSE (your personal situation and goals) should BE considered! LoL