Firebase realtime database is a very good option, but I’m not secure about the prices. If the application becomes viral I think I would pay a lot more than having my own server ($0.06 per auth, imagine in a viral app the costs of them). And another disadvantage is that I do not know the manipulation of data that Google could do without my authorization.
You misread it. $0.06 is only for authentication by SMS - not for Facebook or Google login. Where did you read, that Google can manipulate your data? That would be the end of Google, if it would be public, that they change customers project data. I don’t understand your point of view - everybody is saying “Google is evil”. But they can’t, because their business is build on trust. If no one trust their services, Google will end to exists. - Firebase can create daily backups. It’s up to you to download them and compare it to the day before. This way you see every unauthorised change.
An own server can’t be so secure like Firebase/Azure/AWS. But I wish you good luck and please report, if the first DDOS or hack has happened