"In any case, if you have a Node.js module or code with a native C/C++ build, that runs shell commands, that writes to disk (not allowed besides /tmp in Lambda)

"In any case, if you have a Node.js module or code with a native C/C++ build, that runs shell commands, that writes to disk (not allowed besides /tmp in Lambda) or makes assumptions about the OS, your "simple" function will absolutely return different results." This is true, but it's not Lambda qua Lambda. That's just normal production vs. testing environment issues, with the same basic solutions. Lambda may offer some minor additional hindrances vs. something like Docker, but I wouldn't consider that catastrophic.