Depends what you mean by "extremely difficult". I've had many experiences where the source code was available, yet finding what to change and figuring out how

Depends what you mean by "extremely difficult". I've had many experiences where the source code was available, yet finding what to change and figuring out how to rebuild the whole thing --- but without changing anything else --- was far more difficult than patching a few bytes in the binary. To quote the title of this article, "open-source is just that".