While this is from the point of view of an elemental shaman, most of his points are valid for any class and should be considered. They also cover most of the reasons I don't want to use Nibelung for my gear set.
So with his permission:
Let me first say that "fun" procs don't work in high end raiding.
Marrowgar. When your valkyr stands in the fire, it will die, regardless of the fact that it can heal itself. When your valkyr stands in the whirlwind, it will die. When you turn to DPS Bone Spikes, your valkyr won't help you. You--the best burst DPS class in the game--could have done more damage to that Bone Spike if you had put on a one-hander and shield, but instead, your valkyr is attacking a boss that doesn't have a notable enrage timer.
Deathwhisper. You're attacking a magic vulnerable add, and so is your valkyr. The magic vulnerable add dies. Your valkyr proceeds to cast spells on the nearby magic immune add. It is now doing less than 1 DPS. If you get particularly unlucky, your valkyr may attack and kill a friend who gets mind controlled.
Saurfang. You are THE best kite class/spec in the game for this fight. It is highly unlikely that a valkyr will spawn during your kite, as you aren't casting as often. Prior to Blood Beasts, any valkyr procs you get will continue to attack Saurfang instead of the Blood Beasts. The valkyrs are doing 0 DPS to the most important part of the fight.
Rotface's slime spray will kill your valkyr.
Festergut's plague ability will kill your valkyr.
On Putricide, your valkyr will die to slime, bombs, and mutated oozes. Your valkyr will not switch to kill the green or orange slimes, the most important part of the fight. Should any of your valkyr survive, they will constantly be adversely affected by extreme travel times.
On Princes, your valkyr may get stuck on a Kinetic Orb, or an inactive Prince.
On Blood Queen, your valkyr will die to the fire and the bloodbolts.
On Dreamwalker, your valkyr will be adversely affected by movement, and they will not target the newest dangerous enemies. Instead of having the DPS to kill Blazing Skeletons before they wipe your raid, you will be contributing by having a valkyr, which is attacking a harmless target. --Actually, now that I think about it, the valkyr won't be attacking a harmless target. The valkyr will be dead because of the Blazing Skeleton AOE.
On Sindragosa, the valkyr is a double-edged blade. It can attack when you can't--when you have Unchained Magic, or when you are behind an ice block. It can also prematurely destroy the ice block that your raid is standing behind, and wipe your raid, and waste a limited attempt. In the event that it doesn't wipe you, it will die to the frost AOE and the stacking buffet and the cleave and the tail whip and the frost breath and frost bombs and ice block explosions.
Lich King requires even greater and smarter add switching and DPSing than Dreamwalker. In Phase 1, your valkyr will quickly revert to attacking the closest thing in proximity--probably a harmless ghoul, which was going to be cleaved down anyway. That will increase your Recount DPS, but in reality, it is 0 DPS; it is ineffective raid DPS, on a fight where you WILL die to the enrage. (I did last night; 5% from a kill.) In Phase 3, it is unlikely that you will get a valkyr spawn while DPSing the Val'kyr Shadowguard--so when your friend gets dragged off of the edge of the platform and dies, you can let them know that you decided to give some of your DPS to your valkyr, which was attacking the Lich King instead--or, in a more real scenario, dying to Defile. In Phase 5, your valkyr will not switch to Vile Spirits because of proximity, despite the fact that they will kill anyone they hit--including the valkyr--and need to be killed immediately. You also need the kite the Vile Spirits, so much like Saurfang, it is unlikely that the valkyrs will appear at all. In Phase 5 and 6, you will be phased and killed. Both phasing and death destroys any valkyrs you had up at the time. Now you will have to wait for them to proc again to resume your DPS' full potential.
Oh, and--yes, like haste--they don't affect totems. The difference between haste and valkyrs is that haste helps you win the game all the time, and valkyrs sometimes wipe your raid.
Take your pick.
I also thought of some other incredibly dangerous scenarios:
- Your valkyr may attack frost orbs on Heroic Anub'arak, wiping your raid.
- Your valkyr may attack a CC'd target on Faction Champions, wiping your raid.
- Your valkyr may attack dark orbs on Princes, wiping your raid.
- Your valkyr may continue attacking Putricide even after your raid leader calls for a stop on DPS. This can wipe your raid.
- Your valkyr may continue attacking the Lich King even after your raid leader calls for a stop on DPS. This can wipe your raid.
- You can't be on star or constellation duty on Algalon with Nibelung, because the valkyrs may prematurely kill your stars or constellations, wiping the raid.
- Your valkyr may kill a Mimiron piece out of order, wiping your raid.
- Your valkyr may kill a Freya elemental out of order, wiping your raid.
And as far as I know, valkyrs are unable to take advantage of gimmick buffs, which WOTLK is full of. (source)