Onesignal vs GCM(FCM)

I help work on OneSignal. Here are some reasons that OneSignal could be useful in this case.

• OneSignal provides a single API and management interface for APNS (iOS notifications) and FCM (Android notifications).
• OneSignal tracks notification click-rate for individual messages and campaigns.
• OneSignal provides an SDK that takes care of reliably displaying messages that are received, restoring messages after an app update (on Android), and making it easy to customize notification categories, sounds, LED color, and Lockscreen visibility.
• OneSignal stays up to date with any changes made to the operating system or notification APIs so you get access to the latest platform features without any additional engineering work.

FCM has two types of messages.Notification message which deliver in most cases, and data message which doesn't sometimes. A possible solution is to set priority of data message to HIGH while sending from server side. https://firebase.google.com/docs/cloud-messaging/concept-options