1. Security — I much prefer to hand write APIs rather than using security rules (see recent Arc browser exploit, among many others). Firebase pro’s will call those skill issues, but idk if that really makes it less of an issue.
2. Lock-in - I’m fine with S3 and SQS where the lock in is pretty limited, but Firebase is a floor to ceiling commitment to Google Cloud.
3. Cost - if your app succeeds, you will find yourself investing lots of time moving cost sensitive workloads off of Firebase, though this only matters if you are worried about ‘good’ problems.