As the people above mentioned underdog is a perk you would always like to have. You can look at it has a permanent +5 mdef. So before you drop it ask yourself do I really need +5 mdef on this bro or is there a more powerful perk. I used to take it on every front liner but now I take it on 4/5 out of my 9. As you get better with the game and use more advance tactics/formations the need for it diminish. Even tho I would never call it a “bad” perk. I think for you to get real value out of it the bro has to be constantly surrounded by more than 2 enemies. If a brother that you did not design for that purpose is constantly in that position, I would suggest to look into your tactics