Tink recently outlined the basics of diplomatic units. My thoughts:
When considering diplomats, I view them as mostly the same across the races. The notable exceptions are elven diplos being faster, and dwarven diplos being slower. Orc thieves have a superior carrying capacity. That's about it. The use model for the each class of diplomat means that you will generally have enough attack power to complete the mission, with the exception of scouts.
Most players who get burned on a diplo army have made one of two mistakes.
First, they sent out smaller forces, often to many targets. These smaller forces are more likely to be defeated and killed. Sometimes that's okay or even expected, particularly with speculative assassins, harassment saboteurs, and small scouting parties. At this scale, even runes can be 100% fatal to the mission attempt.
If a large party is lost, it's usually thieves, and almost always because a player got greedy and repeatedly hit the same city. Eventually a player's allies will embed defensive thieves and reinforce. It is easy for them to know the required number to win, because the number of enemy thieves can be directly calculated from the prior robbery reports. So thieves: don't get predictable!
Until next time, my dear cannon fodders:
Misbehave. Kill lots of stuff.
<^^^^^^^^||==O Skint Jagblade