[padho]

I think it really depends on what kind of data is stored. For something like a chat it is a really nice solution in my opionion. If you have to apply complex queries and need full control over your data I would recommend a "real" backend.

For instance, we use Firebase for our community feed on http://spotpot.co but I would not use it to store user accounts there. In the end it is always a trade off but consider that for heavy real time applications a service like firebase can not only be convenient but also save you load on your infrastructure.