[kuschku]

In most cases, notifications might be delayed by several hours, yes.

Or never happen at all.

Or just work normally.

Depending on when the heuristics think you are less likely to use your phone.

There is one way to keep getting notifications, by using proprietary Google services, and sending a ping from your server via Google to the device, or sending the actual notification that way.

Which, for email, means either everyone who does that has to build every app themselves from source, or you have to send that data to a third party server which then sends it to Google