Softer than Uberti, a little bit, and recutting the forcing cone on my Piettas showed the barrel material to be fairly soft, so be careful with the cutter.
Especially on a Pietta Cap & Ball the (almost non-existant) forcing cone needs to be cut properly to shoot a 45LC conversion cylinder with it's harder, heavier bullet. The Pietta or Uberti is probably made of harder, better steel than the original guns, regardless.
I have 2 Pietta Remington conversions and I have shot them in many CAS matches with R&D 45Colt cylinders. There has been no sign of a problem with the throat deforming or splitting, so I don't worry about it.
The "soft spot" is internal, the bolt and hand. A conversion cylinder made of 4140 or 4150 steel with sharp rachet edges (like the Kirst) will chew up the nose of a Pietta or Uberti hand fairly well. We need a source of hardened steel hands for the Italian guns.