On the dawnguard order side we have an Ancient looking order that has APPEARED FROM NOWHERE and has never been mentioned before. Furthermore there are already Orders dedicating to fighting undead.
On the vampire side...
Firstly skyrim's Main Group of vampires are the Volkihair . Who Live (or stay dead, whatever) underneath Frozen lakes. They are basicaly "ice vampires" and so probably look like Drowned corpses covered in frost. Not a bunch of Fancy people living in an island castle (which might be akaviri).
Secondly the bat magic- What a horrible cliche that doesn't fit with the magical rules of Tamriel . You Summon spirits or Daedra- Not small and fluffy animals! It does look nice but so do shiney things.
The werewolf like transformation is also a Non tes thing. It's like they just ripped ideas of a movie to be Epic and hip.
On advancing lore- My anology is something like this.
It's OK to find out about a hand. How it's muscles move and where the tendons and bones are. Perhaps where the hair is on the hand.
It's Not ok to Expand upon the hand by adding Feet to the hand and Removing the fingers. That's pretty much what Dawnguard appears to be doing. Like they Grabbed a load of crappy movie ideas and threw them into skyrim.

