Lamda/FaaS should be use as API or just for image processing

So was talking with a friend about use Firebase Function as our API and he told me that lamda functions (FaaS) should be use only for small operations like image processing, it is not good use like API because we have business rules and will be difficult to maintain.

So.. there are any use case for Lamda functions using like API ?

And about maintain this code (all business rules inside these functions) it is more complicated?