My wife and I tried charcoal (more on curiosity than because of good reason) for a few years with about 30 different plants of a few dozen Nepenthes species and hybrids. We tried a pumice, charcoal, and Sphagnum mix in a 1:1:1 ratio. After two years, we knocked the plants out of their pots and looked at the root systems. The Nepenthes hated it---they had stopped growing almost immediately after the repotting, and we discovered that the roots had refused to explore the new potting material. We repotted the plants into a Sphagnum and pumice medium, and the plants resumed growth.