I have been using scratch pens for over a decade. I use them on parts that I am going to solder together for better adhesion and I also use them to clean up solder joins that are not as clean as I would like them to be.

While quite useful I stopped using the fiberglass scratch pens because of the dust residue. If one of those things gets on the floor and then into your sock and you put shoes on, you get a damn near invisible fiberglass thorn in your foot that will drive you crazy. Happened about 5 times before I completely swore the fiberglass pens off. So if you do use them vacuum the floor carefully afterward and save yourself a lot of pain.

For the moment I have been lucky and have had no issues (true that I don't use it that often), but whenever needed I just keep out the minimum necessary of the tip, say 1mm or less, to avoid those dangerous broken fibres.

This looks really useful. Going to have to talk to the rep.

These are useful. Be aware that beyond Milos' fibre in the foot, the one I bought from a hardware store also warns against the little fibers flipping into your eyes (wear goggles) and being aspirated (wear dust mask).

Yes! I keep a water nearby to dip the scratch pen as I go to keep the broken fiber bits from spreading about.