After having used Yosemite for some time, installed with the Unibeast/Multibeast method, and only having the iMessage issue, i decided to re-install using Clover as various sources point to iMessage fix was easier then.
Actually now it is also icloud that is not working. Not very usable...
Guide: How to Fix iMessage Version 2.1
OSX & AppleID Configuration
Initial Public Release: 25th February 2015
Re-Formatted: May 2017 More Info
iMessage Status: GREEN (iMessage is working with self-generated MLB & ROM Values as per-guide)
Check Apple's On-Line Services Status