if you tried Dawnguard at an early level you are going to get your butt kicked, and the level caps does matter because the bosses and every enemy npc level up when you do so that's why Todd said in an interview or when doing Behind the Scenes he stated or someone stated: "We want the enemies in Skyrim to be more of a challenge than Oblivion, forcing the player to change tactics during combat" therefore being at a higher level DOES in fact help the player by how much is up to sheer luck
Umm no, thats caused by the fact that the area has a set minimum level for monstersm, which is level 10, trying to do Dawnguard before level 10 is only hard becuase the lowest monster spawn is level 10.
The monsters in Dawnguard scale the exact same way as every other monster in Skyrim does. Level req in no way affect the power of monsters, they just make monsters not spawn lower then a certain level. Luck has literally nothing to do with Skyrim's combat/leveling system.