Most of the time, my group messages will fail if they are sent to a group that includes an android. I can send messages to the androids individually, but not as part of a group. I've turned sms and mms off and back on, hard restart, and I've also reset my network settings. I'm not the only one having this problem.
I can send to mixed groups. On iOS 12, these are the settings I have turned on under Messages:
Hope this helps!