That logic works. There are only so many counters and with a barracks level 3 and having 25 active followers plus an inn, you can have multiple followers that counter each trait. If you get a string of deadly minions and got unlucky and only have two counters to deadly minions? Oh well, clear those out of your mission log. But as you ding your followers to epic and each has two counters you'd have to be really unlucky to be lacking in one counter.
At some of the higher ilvl missions, the dungeons are going to have 6 things to counter spread out across 3 bosses, so you're going to want a balance eventually anyway.
There are rare missions that award a book that lets you reroll your followers' traits/counters. Plus you can recruit for a specific trait/counter at the inn.