The complexity bogeyman is a red herring. What is software development if not a constant fight between increasing complexity and adding features, including making systems more robust and resilient and reliable?
Sure, you are free to claim that function-as-a-service paradigm is very complex because the code runs somewhere else, and you have to use a separate workflow than the one you're used to, and that you actually have to write unit tests to have some assurances that stuff does work as expected.
But what would be the alternative approach?
Would it be simpler to write a full-blown service that polls events and listens to message queues and launches background tasks? Obviously, no.
AWS Lambda is a fancy way to let developers develop event-driven systems by just writing tiny event handlers and plug in events. It's no rocket science. In terms of complexity, they pale in comparison with even the tiniest hello world windows GUI app, or even Qt. I'm terms of web services, they are a service where you just have to write the request controller bit after that runs after all auth things passed and request messages were parsed and validated. Considering this, isn't it unreasonable to talk about increasing complexity, when everything was made.far simpler than what it would otherwise be?