I did the standard campaign on FM and the difference to Colonel is relatively marginal. What makes things more difficult is that you cannot afford to use elite replacements for every units all the time. Then it is only about one thing, is your core experienced and strong enough to win a decisive victory in the USA in the end. This is where you basically have to replay the final scenario if you fail till it works out!
The DLCs are an entirely different challenge: Better save up early, from late 1942 the 1943, 1944 and 1945 DLCs will be put you into an entirely new situation, the enemy is not only making your advance more difficult but is capable to crush you if you are not on your toes!
I hit my "FM" limit in 1943. From then you really have to know and play the game mechanics to the max, at least I had to.
It's interesting how Prestige considerations change the game on FM level. For example while Tigers are expensive they usually take little losses and can save Prestige. StuGs offer a lot of bang for the buck. You will also try to back up your Infantry better with artillery and become less reckless.
Whatever - it's a lot of fun and the forum is always a good place to discuss scenarios and strategies!
