XP runes are the most common drop from Brave New World, but they're typically not part of the first time rewards you get for clearing a given floor for the first time, which is probably why you haven't seen them as often. Subsequent runs will more often get you XP runes.
All bosses are immune to being Stunned, Frozen, Taunted, Sealed, Silenced, Blinded, Passive Disabled, and Doomed. That's true for the Dragon Kings as well as bosses in other game modes (such as the Lord of Light from his dungeons, or Aset from Kingdom of the Sands, etc).
As Kardas noted above, the Dragon Kings are also immune to "burns" (which includes all kinds of damage over time effects, like Bleed), and their Force Shields and Titan Shields are special in that they cannot be removed or stolen.
We're continuing to work on best tailoring the best VFX we can to devices based on what we think they can handle, and we felt like we may have overdone the optimizations on some platforms than others. Are you noticing any more or less crashing than before with shadows enabled, or is it about as stable as it was before in that area?