What specific features of a reliable rear derailleur make it a trusted companion on tours, and how do these features mitigate common issues such as chain skipping, ghost shifting, and derailleur hanger damage, particularly when compared to less reliable systems that may require more frequent adjustments and maintenance. How do the materials and design of a reliable rear derailleur contribute to its overall durability and ability to withstand the rigors of long-distance touring, and what are the trade-offs between weight, cost, and performance in this context.