There is a warning on the Firebase best practices documentation against using Firebase with multi-tenant applications: https://firebase.google.com/docs/projects/learn-more#multi-tenancy
This is what I am most concerned about: "Multi-tenancy can lead to serious configuration and data privacy concerns problems, including unintended issues with analytics aggregation, shared authentication, overly-complex database structures, and difficulties with security rules."
There is also plenty of official Google documentation supporting the use of Firebase for multi-tenancy, for instance: https://cloud.google.com/identity-platform/docs/multi-tenancy-authentication .
Do you know why they would have these conflicting recommendations and examples? Does use of Google Identity Platform fix the core security deficits mentioned in the warning?
I am re-posting this question, with additional clarification in the title, and a few edits/removals from the body, to specify that I am only looking for why this widely used product has this particular warning in its official documentation. I have removed most subjective content. I have no opinion on this that is relevant to the question - I am only looking to understand the warning. It seemed there was one good answer before the previous question was closed, so I will link that here for reference: Why is Google Firebase not recommended by Google in their own documentation for multi-tenant applications?
That does make sense if you manage 2 separate applications which have no relation with each other. Let's say you have an app that manages a school's information and other one is a restaurant management app. Now in this case I don't see any event that the school app might need access to restaurant data.
If you use the same project, then all the firebase services (auth, database, analytics, etc) will be shared among them. It'll be hard for you to separate analytics for each of the app. As the database is shared, you'll have to explicitly separate data of both apps by separating the path in db. (/apps/school for school, /apps/restaurant for restaurant).
That being said, any user registered on the school app can login on restaurant app without creating a new account there as you are sharing the same project among them.
Now if your client pays you a the Firebase costs every month, you cannot distinguish between how much should the school client pay. Now even if both the apps are your, the complexity will increase significantly if you go on using it.
https://firebase.google.com/docs/projects/learn-more#multi-tenancy <-- this explains how "Firebase Projects" works and https://cloud.google.com/identity-platform/docs/multi-tenancy-authentication explains about "Google Identity Kit" multi-tenant auth. So that's not a Firebase-only thing.