Jeigans are overrated by Fire Emblem n00bs, and underrated by experienced players.
Some of them, however, like Fire Emblem 4's Cuan can be quite useful sometimes.
Jeigans are often underrated by players who care more about how a unit's stats are at the highest possible level rather than how they are in certain chapters at certain levels.
A Jeigan character joins early in the game and is vastly more powerful than any other unit you'll come across until about half way through. Because they can take on entire armies by themselves using them eliminates the need for prudent strategy involving all other characters.
Player 2: "You shouldn't be using her man, she's a total Jeigan"