Just updated 2 CCR Core Routers to 6.42.12 (Long Term) this night.
One got OSPF Problems with a neighboring router which runs a 6.39 stable version. As soon as the older router is the designated OSPF Router the connected route and all routes behind the older router do not get forwarded into the network. Upgrading the older router helped. So MT OSPF Implementation is not compatible with an older MT OSPF Implementation.
The second got autonegotiation problems on a Mikrotik SFP Module connected to a SAF Lumina with fiber. Setting it to 1G Fixed solved this problem. Lucky I am at the right side of the interface to be able to do this. The older 6.39 version did not show this problem.
So updating MT is still an adventure which cost night time/sleep 
. I avoid it where possible but the exploits force me to do this.