I don't have any problems in the later levels with my other units, it's more the first levels that are difficult (and I don't grind my main army). How much thought the sidequests require depends on how strong you are, it's just that you can't rofl stomp through Donnel's sidequest if you want to recruit him. Let's say: He's a pain at first, then he's really strong and in the end he's a bit weak