All the little devices that want to handle system notifications with email? I can't give them access to my gmail account so they can send, and I don't want notifications in my email anyway If my NAS drive develops a health problem, I want a push and I don't want to find it in my email a day later.
I was thinking about setting up ntfy.sh soon. Of course, you always run the risk of the notification service crashing and not sending you notifications about it, lol.
I think MQTT as a base with gateway features is a good path for this. There's even some android apps the do push natively with MQTT