I have never seen an unsafe aftermarket or original nipple for that matter. Every aftermarket nipple I have ever seen/used was better quality than any original nipple. I recommend you stay away from nipples made by the gun manufacturers like Uberti, Pietta, etc. as you can buy much better. The hole in most aftermarket nipples is smaller than the factory one but no matter what the size they all erode with use. Heavier powder charges and heavier bullets cause higher breech pressures which speed the process. The size of the hole may contribute to or detract from accuracy which is why many shooters monitor the erosion and replace the nipple regularly. I have never seen the need for any of the "hot-shot" type nipples.
Hammer blowback can be caused by one or more issues. Heavy loads, a weak mainspring or a too large hole in the nipple or any combination of the aforementioned. I shot an all original 1861 Springfield and that included the nipple that was screwed into it the day it left the arsenal for a number of years. The hole in that nipple was so large that when I pulled the hammer back to cap the powder was flush with the top of the nipple. My charge was a .41 Henry rimfire case of 3f which I seem to remember was 50 grains. I never had a moment's problem with the hammer lifting or anything else either and won my first first place gold at a national match with it.
Again, the nipple that came in the gun is adequate and safe for plinking and informal shooting. If one gets into punching paper and looking for medals then it needs to be replaced.
If you buy a used gun, reproduction or original, I recommend removing the nipple and examining it and the threads in the breech before firing. I once bought an original underhammer and the first thing I did was pull the nipple. The threaded portion of the nipple had been broken leaving only about 1 1/2 threads to hold it. Had I fired it that way I might have had to remove the nipple from my forearm with a pair of pliers.
