As Byzantium I once hired a unit of Royal knights when I noticed they were led by a member of the Sicilian royalty as he had the family name, which made it interesting as I was fighting the Sicilians in the middle east and I gave him command of my army to recover Palestine.

I can rarely resist hiring Billmen, Italian infantry and Longbowmen as they are pretty much the best of their classes that you can regularly get a hold of.

I recall one time as the Egyptians I was fighting alot of wars and was very overstretched militarily, so mercenaries grew in number within my army and I sent an army made up of Byzantine infantry, Italian infantry, Billmen and Longbowmen mercenaries to Italy to lead the upcoming campaign. They performed brilliantly and I permenantly stationed them in Italy as a garrison as I hadnt any spare natives to replace them and continually replaced damaged units with fresh ones, a few years later though a relativelly minor prince rebelled and although he didnt have all that much support the entire mercenary corp in Italy rallied to him.

I eventually triumphed during this civil war, but Italy and Sicily were devastated in the process and the mercenaries in my army finally crushed by my native forces and I never allowed them to have such a strong position in my army again. Unfortunately my manpower problem didnt end and I struggled to keep up the fight without my mercenary armies.