Hidden inefficiencies

Some people see serverless as a NoOps solution. That's not true. DevOps is still necessary. You don't need to worry much about servers because they are second-class citizens and the focus is on your business. However, adding metrics and monitoring your applications will always be a good practice. It's so easy to scale that a specific function may be deployed with a poor performance that takes much more time than necessary and remains unnoticed forever because no one is monitoring the operation.

Also, over or under provisioning is also possible (in a smaller sense) since you need to configure your function, setting the amount of RAM memory that it will reserve and the threshold to timeout the execution. It's a very different scale of provisioning, but you need to keep it in mind to avoid mistakes.