Actually, the only time I've had them land is with Dragonrend, or when they are at low enough health. Often they will just land (a bit heavily, but no real impact), but if enough damage is dealt to them prior to the landing animation, they will crash to the ground instead, creating a permanent change to the ground (it is laggy sometimes though). Makes sense actually, when hurt enough flying isn't exactly pleasant, and dragons are too prideful to just leave (well, besides that one, but that is a spoiler).
I will agree, however, that they really need to be upped some, or other mobs lowered a bit. That bandit plunderer should not be more deadly than a blood dragon.