Another difficulty was caused by the Doze mode, introduced in Android M. The Doze, which is turned on after a period of inactivity, among other things prevents background processes to access the network. As you can imagine, this prevents our app from receiving notifications.
We mitigate this problem by asking users to make an exemption from battery optimisations for our app. It worked fairly well.
This is why we were migrating to FCM from our own custom built MQTT solution that was invented back in 2012.