There is a heavy bias in this game to specialized units due to a couple reasons. 1) They have extra buffs just for being specialized. Things like +10 Avoid, +10 Crit matter a lot. 2) They get the best weapons in the game. S rank weapons are WAY stronger than A/B ranks, and having access to Brave weapons certainly doesn't hurt either. 3) Honestly, they have amazing distribution of stats compared to other classes. Berserker, Trueblade, Holy Lancer, Holy Bowman are honestly unfair with their stat distribution. Ninjas, Maids/Butlers, and Sorcerers have good stat distributions with very unique advantages. 4) All except Maids/Butlers and Sorcerer get their weapon's "faire" skill. This increases their damage even further than their stats and S rank weapons. Yes, the other classes could steal these, but why be another class when your original class can use this damage increase so much better?
However, some units have stuff to match these advantages. Generals, Basara, War Priestess, and Mountain Priest just have straight stat advantage. Generals are unbreakable walls, Basara's can attack with any option competently, War Priestess can both heal and use Magic Bows to great effect, and War Priestess can heal and do lots of damage with even the B or lower rank Lances. Managamar and Nine Tails have high stats and can manipulate their stats to fit the current battle with great unique skills. On a final note. Most classes in this game aren't bad. It's just that some are exceptional, and a few are less good.