We don’t have to manage a server, that’s really the drop in complexity. We just write a function and it runs on a machine somewhere in the cloud. As we scale, AWS just handles it automatically. If the demand decreases, no problem. We just pay for what we use. Of course there are ways to handle scale with servers, but with serverless you barely have to think about it.
The only thing that comes to mind better isolation between parts of the app, but this could be achieved with any architecture if done right.