It's not THAT difficult to kill people with werewolves at high levels, I imagine the vampire lord will be the same way. If you're a high level all you have to do is avoid transforming in front of powerful enemies and large mobs.
That will just make the leveling slower, you need to kill the foes with life drain to level up the vampire which will be much more easily done when your low-level as well. Werewolf would be a bit easier as you could wipe out the dungeon transform then feed on everyone you killed, but they might check for that tactic.
I have to disagree there is plenty of people to kills in skyrim, plus the game respawns the camps after a set time
Ah but you'll be facing higher level enemies using lower level abilities from the respective perk tree.
It could be like switching fighting styles mid way through the game from a two-handed heavy armor to a destruction using light armor. (though without the wasted perk problem) Is significantly more difficult as your foes are scaled high but your using low-end skills. And given we know so little about the vampire and wolf abilities we can't be sure what exactly is going to be advantageous.