Depends on the definition of gamebreaking.
I posted a while back about a bug that's reared its ugly head in my Iron Man games twice now. Both times, my grenadier was the one who took the last turn and shot a grenade at an enemy on higher elevation, destroying the ceiling and dropping the alien to the ground. On the alien turn, the game was frozen with a background sound of something falling and nothing happened even after waiting for 10-15 minutes. I tried everything. Restarted the game and my PC, checked online for something similar, but ultimately I gave up and restarted the game. This was when I had 20+ hours invested in the game.
What should I be prioritizing in the Geoscape guys? What about research wise? And are there missions that I can skip? The retaliation missions, specially the second oen with the muton have always bene brutal, can I skip it until I atleast have armor or magnetic weapons? What the penalty for skipping missions like that?
I always try to get the first three contacts as soon as possible. Research-wise, I go for Modular weapons -> Hybrid materials -> Alien autopsy -> Officer Autopsy (for Proving Ground) -> Mag weapons. I can usually do this BEFORE mutons show up in the game. Once you have mag weapons, you can't really screw up research wise, as long as you get Resistance Comms up and running right after Mag weapons research because you'll be needing more supply drops soon. Remember to tech up to Plasma weaponry as soon as possible by doing Elerium research.
If you feel like you have an underleveled squad for a retaliation mission, go on the mission regardless, but once it feels like you're about to get wiped, call in the evac (I don't know if this works, as I've never had to call in the evac in a retaliation mission). If you absolutely don't have the squad strength to do retaliation, skip it. You only get penalised with a significant, but not too big, decrease in supplies that that particular area would give and you lose contact with them. You can easily make contact again and the decrease in supplies isn't too crippling if you plan your facilities in avenger to get the maximum no. of contacts available ASAP.