I looked through the docs and have some issues with it.
First of all, it is a service. That opens a can of worms on its own. The main issue being that you are 100% at the mercy of the service provider. Every time they decide to change their API, it adds maintenance work to your project. Sometimes it happens on short notice which can be very annoying.
Second, it is a strange mix of service and code. I don't see an easy way to use it without their Javascript SDK. And installing that SDK looks complicated from what I see.
To use it without the JS SDK, you need to create a server which integrates with their API. This usually means pulling an SDK in on your server which is often okay, though I did find their Go SDK (https://github.com/firebase/firebase-admin-go) rough on the edges. We ultimately abandoned using the service... I think if your integration requires any fine tuning or integration with other services, you might want to look elsewhere.
First of all, it is a service. That opens a can of worms on its own. The main issue being that you are 100% at the mercy of the service provider. Every time they decide to change their API, it adds maintenance work to your project. Sometimes it happens on short notice which can be very annoying.
Second, it is a strange mix of service and code. I don't see an easy way to use it without their Javascript SDK. And installing that SDK looks complicated from what I see.