I have 7 different Pietta 1851 Navy .36 type pistols. It is well known in these circles that the Ubertis are known for a short arbor/deep arbor recess in the barrel lug, which if not corrected will cause the pistol to shoot loose eventually, sooner than later. If you use the wedge to adjust the barrel/cylinder gap, you have this problem.
Piettas do not have this problem.
Insofar as caps/nipples are concerned, it depends upon the brand/size caps you use. If the cap fits snugly on the nipple without pinching it, you are good. If it fits looser than that, you can slightly file a bit from the nipple ends to make the nipple larger in diameter, thus the cap will fit tighter. If you can seat the cap without using a small wooden dowel to push it firmly onto the nipple, it is too large for the nipple.
If you want different nipples, go to
https://www.trackofthewolf.com/
They have just about anything you need.
I have never have had a hand or spring break. If you want the coil spring (ala Ruger Old Army) and are concerned about it, contact Mike (Goon) at
http://www.goonsgunworks.com/
He does many good things for replica 1851 Navy revolvers, especially Ubertis. To adjust for the arbor/recess slack he does the modification to the recess as opposed to the arbor length, so you can use any other barrel you have for the gun modified the same without changing the arbor length.
He will completely go through your gun and do the modifications you wish. I have never heard of a single complaint about him or his work from anyone. He will also install a cap rake so the spent caps fall outside the action. He will tune the action so that the timing is right with the bolt to the cylinder. If your mainspring is too heavy, he can cure that also.
It will possibly cost as much as you spent for your gun, but IMO well worth it.
If you are able to catch him on the phone, he will talk your ear off, which is a good thing, but it takes him away from doing work on the many guns he has on his backlog. He is that good.
I am in no way associated with Goon and his enterprise.
Just my $.02 worth.
Jim