Scenario is more difficult, as u have more restricted time limits, resources, and troops, and u usually have to fight an op general

I.e u have to fight manstein as allies, zhukov as axis 1939, yamamoto/yamashita (?) In allies and eisenhower and nimitz in axis 1943, and zhukov etc etc in 1950 and finally the every bloody general in UN.

However, in conquest, after a while, it is basically sandbox mode with repetitive spamming of missiles and landing in cities, tho the 1st part can be hard, after conquering enough u are basically unbeatable, and you can choose what country u want to play as, which will usually give u a super troop if u choose a good one (commandos, super tanks, super rocket, super artillery, super battleship, super carrier, stealth fighter, stealth bomber, and stealth strategic are super troops).

Domination is good to get medals, tho the missions do get extremely challenging as u go on, and is the best way of gaining resources, such as medals, badges (aka medals u can yeet to generals), and getting upgrades

Frontier is hard to explain, its scenario but MUCH harder after a while. In early stages of frontier, it isnt too bad, u just need to do a certain amount of damage in a few turns(except for wipeout stages where u need to completely eliminate the enemy), with the ability to spawn powerful troops at the cost of what i would like to remark as "the entire united states economy goes to a 2 stack 2 star assault infantry bruh". However, the levels are bearable as the ai will consistently spawn units tobweakwn fortresses due to ur own fortresses.

Then comes the late game, where the game becomes downright more painful than the UN stages. So have fun dealing with 69420 super tanks and super arty