What specific considerations should be taken into account when upgrading a traditional mechanical groupset to an electronic groupset, specifically regarding derailleur compatibility, and are there any recommended compatibility charts or resources available to ensure seamless integration between different manufacturers components. Furthermore, how do the different electronic groupset systems, such as Shimano Di2, SRAM eTap, and Campagnolo EPS, differ in terms of derailleur compatibility and what potential issues may arise when mixing and matching components from different systems. Additionally, are there any notable differences in derailleur compatibility between road and mountain bike electronic groupsets, and how do these differences impact compatibility and performance.