No, not overpowered. Just better than levy units. A good metaphor would be every faction starting out with many units of Militia Pikemen and Light cavalry, with a few units of Silver Shield Pikemen and Companion Cavalry mixed in, but only the ability to retrain the militia and light units initially. So if you lose your Silver Shields and Companions fast, you'll be at a disadvantage if the enemy still has theirs. They're definately not invincible or overpowered, though.