Traditional selfbows don't usually have an arrow rest and the arrow is shot over one's hand. This means only bird feathers or similar soft fletching can be used with their arrows. Crossbows don't suffer from this limitation, so the fletching can be made from basically any material.
Metal vanes are sturdy, cheap, quick to make and easy to fix, should they get damaged. Thin (0.5 - 1.0mm) sheet metal (copper, brass or steel) will work well and metal-vaned bolts seem to be relatively accurate at normal shooting ranges (up to 40m). Metal vanes are probably not the best choice for really long-range shooting where absolute accuracy is required: the vanes have a tendency to bend slightly when they hit a hard target. While straightening is easy, the bolt will no longer fly 100% the same as previously.
Attaching metal vanes to the shaft is somewhat slow, but their biggest downside is their weight: a metal vane can easily weight 5-10 grams, changing the balance point of the bolt significantly. Also, as the vane is as heavy as it is, it has considerable amount kinetic energy. This means that the vane can easily split the shaft if the bolt hits a very hard target (=decelerates rapidly). There are a couple of precautions that can be taken to prevent this:
- Glue the vane to the slot in the shaft so that it can't move during impact, unless the glue joint or wood gives in. Epoxy or similar glue with separate glue and hardener works the best, even though it's far from "traditional". Remember to wedge the vane to the shaft with a small piece of wood or metal during gluing.
- Bind the shaft directly in front of the vane with strong string. This won't prevent the vane from moving forward during impact, but it will prevent it from splitting the shaft. Adding bindings behind the vanes would helps prevent end of the shaft splitting if the bolt penetrates a relatively soft target.
These approaches can be combined for best results, but in most cases using either one is enough.
Regardless of their shortcomings, metal vanes are a good candidate for large bolts weighing 50 grams (~770 grains) or more.