There were some Confederate Napoleons made from cast iron but none of wrought iron (that I know of) and no Union cast iron guns (again, not that I know of). Parrots were made of cast iron with a band of wrought iron around the breech but a better technique (Reeve's process) later allowed the Ordnance rifles to be made of several bands of wrought iron which was lighter and not as brittle. Not too long after, steel started to become the standard.
There is some disagreement about whether the field pieces were really that bad but there are photos of heavy Parrots with burst barrels. Many think those guns gave all Parrots a bad name with a few officers. The Ordnance rifle certainly became the favorite.
The 24 lb Bronze Howitzer was part of the pervious mixed howitzer and gun batteries of the prewar period, the complemented the older "Heavy" Model 1841 12 lb Bronze Guns that the "Napoleon" Model 1857 Gun-Howitzer replaced. It was a fairly rare weapon as I recall, because it was meant to compliment the old heavy 12 lbs that were also not produced in great numbers. I think there were also 32 lb M1841 Howitzers that were also out there, and also accompanied the heavy 12 lbers on occasion. All these guns were pretty cumbersome, as I recall they required 8 horse teams to move and were not easy to manhandle.
The Iron 24 lb Garison and Siege gun was not part of the field artillery, but rather part of the Army's "siege train"... they were just about the heaviest guns that were road transportable and could move with the army. Actually, most have been made as seacoast artillery guns, but were largely obsolete for the purpose of arming forts against ships and so were relegated to the siege role instead. Most of them were older guns, from before the copper boom in Michigan made casting bronze guns cheaper. The Army needed alot of them to arm forts, and American foundries were more capable of casting iron, so they opted for an all iron artillery system starting in 1819 (although honestly most US cannons before that were also cast iron, so in some ways this just codified the status quo). Cast Iron guns in larger calibers at that time seemed to do OK (perhaps because they had more metal and thus strength in them) but on the field artillery end of the spectrum the Army had alot of problems with bursting due to metal casting defects.
As others have said the Napoleon design eliminated alot of the rings ect from the exterior of gun design because that added areas of weakness. Although those rings were somewhat decorative, it is my understanding that previous designers had actually thought such bands\rings along the gun added strength, probably on the basis of very early cannon designers experience that was carried on past relevancy. As eraly as the system of 1819, the US was starting to realize that wasn't the case... only when the Model 1819 family with it's smooth exterior lines came out, it gained a reputation for bursting due to the aforementioned flaws in metallurgy. Quickly the design shifted back to more conservative exterior forms to "correct" the "problem". Gun design right before the war was actually already moving beyond the Model 1857 "Napoleon" towards the "Coke Bottle" shape that the Ordinance Department had decided was the new ideal of gun design. The latter shape can be seen both in the 3in Ordnance Rifle and some "Confederate Napoleons" that were made in iron, and with the new shape in someways represented the cutting edge. Technology during the War was moving a mile a minuet, and so the state-of-the-art and the obsolete frequently coexisted, especially in theaters outside the East.