I used to work on the Delphi compiler, for 6 years or so. That compiler was not a simple function of file input to file output. It lived in an IDE. It had to do incremental compilation, throwing away the compiled version of an in-memory file and recompiling it, but avoid recompiling the whole world of downstream dependencies if not necessary. It had to provide code completion, which meant parsing and type analysis of a file, but skipping codegen, with reasonably sophisticated error recovery. It had to provide the debug expression evaluator, which lived intermixed with the constant evaluator, and needed to know how to set up calls to symbols in the program being debugged. And it had to do all of these things in a long-lived process, with no memory leaks and high reliability.

The degree of asynchronicity is low, I agree. The most significant was just interrupting compilation on user request. But the complexity was not trivial.

These days I work at a startup on a full stack SaaS; C++, Java, Rails, and Coffeescript with as few blocking browser actions as possible. It is far less complex than compiler work, but it pays better. I've never had to debug OS code to diagnose runtime library issues in this job; nor write manual structured exception handling. I haven't had to try and build stack frames dynamically to implement reflection calls, nor unpick stack frames dynamically to implement virtual method interception. The relative complexity of potential races with SQL read-committed, or writing UI such that it can't become inconsistent, really doesn't compare.