FCM: Is data message delivery really less reliable than notification message delivery?

Question

I have come across some voices stating that FCM data message delivery is less consistent compared to that of notification messages. Does anyone have direct experience or can point me to resources exploring the issue? Or is a notification message just a collapsible, high-priority data message that the Firebase SDK handles automatically?

• https://stackoverflow.com/a/49998176

• FCM data message not received in Android when the application is in background

• About FCM's notification message and data message type, which has better receiving rate

The question does not consider the case of force quitting the app. In this scenario, both types of messages will not be delivered (to my knowledge).

Background

I am writing a new Android SDK for a push service provider (similar to OneSignal). The SDK should handle the display of push notifications by default, optionally the client app can handle incoming pushes itself.

The actual delivery is of course done by Firebase Cloud Messaging (on devices running Play Services). So there are 2 types of messages to choose from on FCM: data vs notification messages.

As data messages are consistently handled by the registered FirebaseMessagingService (provided there is no notification key in the payload), this should be the way to go for the SDK. [See documentation] So far, I have not been able to produce a situation in which a data message was not delivered (foreground or background).

from FCM: Is data message delivery really less reliable than notification message delivery?