> With Firebase, when doing complex calculations, not only does the data have to leave the database, it often reaches end users
Any time sensitive data reaches untrusted users it is a bad practice, but this needn't be the case on Firebase just because it is serverless.
Firebase cloud functions can be triggered as a response to document saves (or authentications, or a plethora of other things) and run outside userspace, and are available for exactly this sort of work -- keeping the minutiae of upkeep away from the client.
That said, I agree that it's easy to make this kind of mistake with serverless development, but mostly due to lack of existing domain knowledge. It's trivially easy to make similar (or worse) mistakes without a serverless environment by untrained developers, too. It's just a matter of becoming familiar with the toolset, and because serverless tech is newer, fewer are as familiar with them as they are the old things (and there are fewer people to catch those mistakes when they see them.)