You can think of firebase in the context you mean here as BackEnd as a service.Giving you database, Authentication etc. It means you have little control on how you'd write many part of your code. You have to do it Firebase's way. It has the advantage you don't have to manage your own servers etc.
Fully agree with you.
Choosing a stack depends on many factors like knowledge, costs, app features .. etc.
Firebase, for sure, is a great platform and helps a lot but from my point of view has some drawbacks:
After using the platform a while, I decided to move away.
In the same time, I'm not advising anyone to do the same.
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