So, I'm actually thinking the consensus is wrong and the light cav cultural retinues might be quite awesome. The east european defensive more than the spanish offensive, but both pretty good. What everyone is forgetting is tactics.
Light cav on it's own has pretty bad stats. And when it's grouped with other units it doesn't get any real bonuses. But when it's in a group alone, it can take advantage of some very good tactics. In every single phase of battle.
In skirmish, it can get a 300% bonus. And then in melee, it can get another 240% bonus. And in pursue, it's obviously amazing.
So if all you have is a light cav army your stats are:
Offensive (Spain): 4.8/5, 11.52/3, 10/8
Defensive (East Europe): 3/8, 7.2/4.8, 10/8
Now, that's pretty decent. That's pretty effective in every single phase of battle. But here's what makes it better. It's every single unit of your army. It doesn't shine in any one phase. So an all cultural light cav army has decent killing power for every unit in every phase of battle.
Think about how that compares. Longbows are amazing. But only in skirmish. They sit around in melee doing nothing. And you can't mass them because they get wrecked without melee units. So as good as they are in skirmish, half your army is going to be something else that's sitting around doing nothing in that phase. So while the longbows shine during skirmish, at any time half your army is going to be sitting around doing nothing. While every single light cav unit is working during every phase, none of them is every sitting idle.
Same goes for heavy cav. First, they're so expensive you'll never have a lot of them. And then come with light cav. Which because they're massed with heavy won't be using the right tactics, won't have cultural bonuses, and will be less effective.
Same goes for heavy infantry. They sit around doing nothing during skirmish. And you pretty much have to group them with horses of some kind or they'll rout their enemy and then kill none of them in pursue.
So I'm thinking big groups of cheap light cav cultural retinues might be pretty darn good after all. You can mass them into big groups. They can win battles plus assault fortresses at an affordable cost. They're pretty cheap. But they're only good if they're massed without any other unit type so they're using the right tactics at all times. I'm going to try a Hungary game at some point to try it out.