Yuri's Epsilon Mission 11: Singularity is definitely the most difficult Mental Omega mission and in my opinion probably the trickiest Command & Conquer mission available and I've played almost all of them (fan-made maps included of course). Coming from one of the best Red Alert 2 mission designers lovalmidas this mission requires really good micro skills but also great reflexes and is a mission unlike any other. The changes include fixes like removal of the 2nd debug Malver which we forgot to delete and also removal of the "cheap shortcut" to the Iron Curtain which allowed players to finish the mission much faster than indended. Now you really need to get rid of all the targets and also evacuate your troops into a newly created area.
I think Allied Mission 11: Panic Cycle is the easiest of "the 11s", however its first iterations were really difficult with Soviet Scud Launchers trying to take out the Battle Fortress you're supposed to escort on the move and large waves of enemy units coming straight at you during the base-vsbase phase. There've been some changes that you might notice in this playthrough, including China's use of EMP against your convoy for example.
On the other hand, while this might be the 2nd most difficult Mental Omega mission, Soviet Mission 11: Unshakeable has been made easier on all difficulties and its bugs have been fixed as well in the process. The Future Tanks will no longer fire at each other when you attempt to capture them and modifications to the array of capturable buildings in this mission will force you to use Chinese equipment instead of Russian or Pacific Front's. You still need to rush quite fast through enemy forces and reach your objective before it gets blown up and you fail the mission.
Once we're done with Act One and the 12th missions are updated, recorded & uploaded we'll begin revealing new content related to the still mysterious Side 4 and the Act Two campaign. Stay tuned!