Serana- She makes your vampire lord weaker. If your a vampire most likely you'll want serana as a companion so why would they make her decrease your powers? Also, She can become a follower no matter who you join and you can still become a vampire lord while siding with the dawnguard.
Deathhounds- They die very easily when your vampire lord uses vampiric drain. Again its a major gimp for a vampire lord because anytime you transform you have to make sure there out the way before you cast your spell. Since they only use melee attacks I don't see that happening.
Mission- Joining the vampires made me think they would be organized,smart and loyal. All they want to do is stab each other in the back and they benefit my character very little. There's no vampire exclusive followers but the dawnguard has plenty! The worst part is that my main goal is to stop a vampire.Really? I could've joined the dawnguard for that.
There's a lot more issues I'd rather not touch on but I just wanted to know if anyone felt the same way I did.I mean I love what they did with the appearance,perks, and vampire lord but it seems Bethesda favored the good guys....again. It's black and white and that's something bethesda said they would stay away from.

I mean it would be nice if vampires got the ability to summon different variants of gargoyles, and to have different companions. We do get jewelry that boosts our VL powers and the blood of the ancients so there are some benefits to joining the vampire side. I guess maybe if the only way to become a vampire lord was if you joined the vampires would've created a much better incentive to join the vampires.