Limit on the number of troops per hero.

most often, I (like probably everyone else) give other heroes troops from different castles. The balance doesn't get ruined, and... it's a backup.
Periodically, I combine these troops to give the main hero all the best.
And sometimes, I give one hero some troops that are strong on their own, while another hero, for example, mainly relies on magic, or uses almost unused melee units to break through mobs and clear the map.