Huscarles are indeed very overpowered units in VI.

So what I did was increase their cost from 475 florins to 575 florins. This will make them more expensive to recruit by me and the AI.

In addition, I made them buildable as mercenary troops. This will help when I play one of the weaker factions. Not many show up, and they are expensive (about 400f to purchase, with 150f upkeep). Even so, it is good to know that you might be able to recruit one or two Huscarle mercenary units that will help out when the Mercians hit you with 5-6 Huscarle units in an attack. At the very least they will be units of last resort to help stop the "Sherman Tank" of the ancient period.