Redis modules offer flexibility and speed by providing access to lower-level APIs compared to Lua scripts, but they are more challenging to maintain and distribute. Modules can add new commands that execute arbitrary C functions, which can be useful for solving coordination problems or adding small additions to existing commands. When designing a module, it's essential to consider data type implementation, error handling, and API polishing to ensure usability. The goal is to create a simple yet effective solution that complements the existing Redis ecosystem without sacrificing simplicity or ease of use.