I think carbon rollers are better than sliding contacts.
I have seen a number of transformers with physical wear on the secondary windings from the sliding contact moving back and forth over them.
Transformers with carbon rollers do need to be checked from time to time. I do not recall seeing a transformer with rollers that had any wear on the windings as long as the rollers weren't worn out, and were turning,
If the roller is allowed to wear to the point where it's holder is hitting the windings, then wear on the windings will occur.
