Why is Google Hangouts so bad?

Every time I get a message from a friend, it shows up late in my android. Many times I never get the notifications. These are very basic use cases and they are so annoying even for a casual user like me. Thankfully I don't use it a lot for serious work.

Is Hangouts being so bad the reason for the popularity of Slack? Is there any specific technical reason for why this software is not upto the standards their other products have?