I use Firebase Database but keep it arms length by avoiding any of the proprietary features it offers. I use it exclusively as a JSON store which I feel I can easily export at any time if I wanted to switch to another NoSQL DBaaS.
A few people here are mentioning how it's crazy to depend on external services for an app, but as an 'indie' dev, I can't spend time maintaining a secure, high-availability database. Or a mail server when I can use SendGrid, or a logging platform when I can use Sentry, or any of the other service my app depends on.