Those that are good at these type of games will beg to differ but there have been plenty of people having trouble with some of the missions (Mission 4 notably, where you try to turtle up against an almost unending onslaught.)
Mission 4's difficulty will be adjusted in the v1.01 patch expected in the coming days. :)
I'm not sure that the problem is with Mission 4's difficulty.
When I first ran the mission, I dropped the speed down one notch to 'Slow' after the first probing attack wave just to make sure make sure I could casually keep up with all the expected assaults - I couldn't and was eventually overwhelmed just before I could make enough Turinium to win.
I then tried again, and then turned the speed down as far as I could to 'Slower' so that I could micromanage every action - once again, I was overwhelmed and realized that somehow I managed to do worse than before (I was now also thinking 'Damn this is a hard mission').
Figuring that there was something I may be overlooking in my tactics, I watched a video of someone finishing the mission completely where I noticed the guy had more units in play to fight with on the first wave than I did when I micromanaged. But I did notice one important key that I did overlook - his tactics were near identical to mine (we both were following the mission script) had set his speed to 'Faster' and sometimes jumped to 'Fastest' (That's when I realized the problem!).
My next game, I upped the speed to 'Faster' and sometimes to 'Fastest', and discovered that the mission just became a cakewalk!
This led me to the realization that the mission script was running on 'real time' (just like the timer by the game speed setting) and not on 'game time', translating into you being able to have more game time (and build a bigger force) by just running the game at a faster speed.
Realizing this major flaw, I started my next mission on 'Fastest' to get a starting edge, (even though you start with the two squads that have to be used to capture at least 3-4 of the first Turinium generators required to give you enough time to build a big enough force), I got caught up spending most of my 'game time' reading and waiting for the mission dialog to slowly complete as I watched the computer generate most of it's Turinium during my dialog time where I couldn't issue commands. Needless to say, I realized that this mission needed to be started on 'Slower' so that the computer didn't get the free or extra Turinium generation time in game time, while I read and tried to click through the mission dialog. Once again a problem with 'game time' versus 'real time' only extremely highlighted by the mission dialog which prevents you from controlling anything, but the game (and AI) keeps running and killing you in the background.
To summarize, the two items that need to be fixed are...
1) Synchronize the Mission Scripts with the 'Game Time', and not 'Real Time'. Game speed shouldn't be a difficulty setting - especially when setting 'Slower' = 'Hardest' . Possible enhancement could also be to add a 'Game Time' clock in addition to the 'Real Time Lapse' clock, or even better put the Real World Time instead of the Real Time Lapse.
2) Pause the game time while the mission dialog is on the screen. There's no reason why you should be penalized (or lose a game) because you were trying to read and enjoy the story.