What I found was that the game doesn't really do a good job at teaching you what kind of units you'd need to deploy for a mission and the differences in between them.
At least in the single player I am just guessing every single time what I'd need for the missions. Sure more expensive units are better but until you play the game a lot and learn the differences between units and find the few Goldilocks units the game can be frustrating.
Also if you unlock the wrong units throughout your career you can get stuck on a mission. That is where I am at at the moment.
I've tried different combinations of units but no luck. It looks like some of the missions are off balance or something. Some feel easy some really tough. I especially hate those get here within a certain amount of time to help these units, type missions.
Anyways I did not mean to rant I am just a bit frustrated with the game and was hoping the sequel would be somewhat better in that regards but by the looks of it I was the only one with issues like that and instead of making the unit selections a bit easier they've tripled the available units so it will be even tougher to find the right combination of vehicles and planes for a certain mission.