There's a difference between a lightweight transpiler like JSX and a compiler.

ClojureScript is pretty lightweight, but GWT and Scala.js compile process is not an easy one. Even Bootstrap.js optimization with Google Closure Compiler is an uphill battle. Try it with --compilation_level ADVANCED_OPTIMIZATIONS

Sourcemap does not help much with compiler bugs or if the compiler people have different opinion from you on how something should be translated.