Lack of diversity in unit types? Are you talking about the Renaissance, the 16-17th century, or the 19th century?
Probably the closest we've come to having few unit types was the period from the Civil War to early World War I, when fast-shooting rifled weapons basically wrecked cavalry's effectiveness. You had your basic infantry (of different qualities of course), elite infantry (some were better than others), cavalry (used only as scouts), machine guns, and a variety of artillery. So that's not too many.
But in the Renaissance & Napoleonic times there were a large number of troop types, who fought and acted very differently. In fact, this era probably had the MOST diversity of troop types within a given nation's armed forces that had been seen since before ancient Roman times.
Consider - in medieval times, the English army was basically composed of knights, a rabble of footmen, longbowmen, and (mostly Welsh) swordsmen. That, plus some siege engines. That's not much. Sure the medieval era had a huge diversity of soldiery, but not within the same army! For instance, the Magyars had horse archers & light cavalry, types which the English army lacked - but that's pretty much all the Magyars had. So the English had four types of soldiers, and the Magyars had two, the Arabs had about four, and so forth. Now in the Napoleonic wars, the English army fielded heavy dragoons, light dragoons, hussars, light infantry, line infantry, grenadiers, riflemen, and a half-dozen or more types of artillery. That's six types, just for them and not counting the variety of artillery, which was much more common in battle than siege weapons like catapults or rams. And the English army was one of the least diverse armies of the war. They didn't even have lancers or cuirassiers.
Then in the 19th century, diversity drops again as I mentioned above - pretty much you only have infantry, scout cavalry, and some artillery weapons.
Of course, nowadays we have evolved back into a state where we have a huge variety of soldiery again. The US alone has 13 distinctly different types of rifle-armed infantry, and our army is disproportionately small compared to that of many nations.