I agree with you that the many flavors of Haskell are problematic when it comes to uniformity. C++ and Scala have similar challenges. The old trope of every C++ shop using their own unique subset of the language is pretty relevant here. One thing in Haskell's favor though...with the language extension paradigm you have a very clear and tangible path to enforcing uniformity in your codebase that you don't have in C++ or Scala. You can use simple code analysis tools (I believe hlint already has this functionality, but just grep is sufficient actually) to enforce your team's choice of language extensions.