Arnuz: Turn sent... I've just discovered looking at a stack of a highman saint that when a unit levels they become more difficult with moral, damn, hopefully I don't lose my charmer right when I need him.
A unit is harder to bully into submission the stronger they are, so medals will indeed make a hostile unit harder to keep under control. Charm/seduce/dominate/etc give experience to the acting unit when they succeed (and there is an etiquette rule against repeatedly charming and disbanding a unit to farm experience).
On the bright side, the charmer gets a little more durable with medals for those failed charms, and they can only get two medals so there is a cap to the increase in morale difficulty.