iMessage suffers from the same interoperability problems that most Apple software does -- they don't really consider use cases that don't involve Apple.
Hangouts for example understands how different carriers handle different types of data differently, and accounts for that when sending messages. So when you send to a group with different capabilities, it adjusts accordingly and automatically in the background.
What exactly is your issue with iMessage? Messages on iOS also automatically adjust to the participants' capabilities. Have you used iMessage recently? It degrades quite well. You can't backfill functionality that doesn't exist on SMS.
I use iMessage every day. Most of my friends are on iMessage. Most of my family too.
The problem is our family group text. Some of the family is on Android. For the family group text, I use Hangouts because not all the messages go through if I use iMessage.
Hangouts for example understands how different carriers handle different types of data differently, and accounts for that when sending messages. So when you send to a group with different capabilities, it adjusts accordingly and automatically in the background.