As others have said, your units max out relatively quickly and after they're maxxed out all the xp they obtain is rendered useless. This problem is highlighted with your hero unit. Im just about I've only played as Viegraf so far but in the later stages, having him fight is a liability since he can get killed really quickly at times. He was really strong for the early levels but I think the heroes need to scale better throughout the game because
"Replaced with better vehicle. Which currently has 10% of the gasoline capacity, no weaponry, drives half as fast and couldn't limb a hill with mountaineering equipment. Also, your earning capacity is now halved and upgrade cost is tripled. UPGRADE COMPLETE!" :P
"Aw man I ran out of fuel! No worries tho I'll just instantly get back to the garage and try again from there!"
 "Epic War 5"){kind=icon}
 "Earn to Die 2: Exodus"){kind=icon}