I've noticed this with the Byzantines and, to a lesser extent, France. Most armies I've fought are fairly diverse, often very dangerous, with me on the defensive. The Byzantines, however, seem to love their horse archer line. I'm not saying it's a bad army at all, but after I destroy their first army or two of diverse units in the field, that's all they put out -- sometimes with some siege weapons or a token Byzantine spear unit. But by God/Allah, their field armies have ten or more horse archer units, period.

And maybe France just glitched in my game. I slaughtered one of their main mixed armies as the HRE. The following few turns, after I'd recovered and went on the offense, they sent armies of...peasants...led by one or two knights of some sort. The peasant armies of France dealt some serious damage to my veteran, high tier armies before the siege bug cropped up. I was contemplating quiting that game just on the peasant bit alone. It's happened similarly in a Venetian game and as the Turks (spies saw them).