I used a fiberglass rod yearsa go for loading some tight bullets in a fast twist Bauska barrel I had. The 3/8" diameter rod was great for loading. I also had to re-crown the muzzle every 300 shots - about once a month sometimes 2 months. I'd watch the groups gradually measure from 3/4" out to 1 1/2" at 100 yards, then re-crown, back down to 3/4". Just one of the experiments I did - way back when.
I-too have just recently built 2 stainless long rods for cleaning my Sharps. I used one for a range rod the other day and will stop that practise immediately. For the Sharps, I have a collar that fits into and protects the chamber and throat, so the stainless rod is a go- as long as I keep it straight.
The two volumes I have, titled Firearms of the American West - note in a couple places, the rod wear and other damage caused by the loading rods used in the muzzleloaders. "Other" damage noted is the 'pounding' against the rifling down inside the bore. One must use care when loading. The cupped ends on the rods, and the habit of slamming the rod down on the seated ball to expand it as needed in the Tige and Delvinge would indeed damage the rifling down inside the bore. This slamming of the rod actually came over to the rifles due to being a normal loading ritual for the smoothbores - that of slamming the rod 3 or more times against the seated ball and ctg. I've seen a number of people on the range at Hefley doing the same with their round ball guns. I've never seen any of those people in the winner's circle.