Interesting question, Tim/Steve has a point with the spell and also with Ghouls. From a 7th edition perspective ghouls ruled the roost with Vampire Counts as core troop choices, and it can be said a strong argument to continue such a list in 8th.
However a Zombie Horde apart from being rather fluffy, does look incredibly awesome when painted well.
My impressions so far on how to use them;
Fear and Terror have been nerfed in this edition, but don't lament just yet the ability of a zombie in melee due to his reduced fear capability. A zombie horde has several factors that do boost his paltry fighting skills.
a). He does cause fear; before a BLOW is struck, your opponent must pass a leadership test. As 8th edition is fairly new most players are still on the BSB reroll bandwagon and well I have 1000000 ranks and my leadership is going to be 10. In theory they are right, but it does not always work this way, in fact, often it does not.
The Vampire Counts generals and hero characters make VERY short work when tooled correctly of a BSB. Most armies are limited with their BSB, they can either give him a decent banner or magic items, but not both. Forbidden Lore allows you access to sniper spells that become deadly against a BSB and a general. Lore of Death, Metal, even Fire can all effectively kill a BSB or take him out if the enemy does not dispel.
Working in synergy your humble zombie can be powerful as soon as you reduce the general to a bloody stain or remove the BSB to start forcing fear tests. On top of that, the VC give you several options to increase the risk of their fear failure.
Aura of Dark Majesty will reduce leadership by 1.
Doom and Darkness will reduce leadership by 3. (From Lore of Death with Forbidden Lore)
(You will find other items that will reduce leadership as well).
Lets say a unit of halberdiers are about to smack into your zombies unit. They have an unmod leadership of 7. Let us also assume they are within 12 inches of the BSB. They have to roll under 7 to strike at you normally. If they fail this test, they get to reroll again due to the BSB. However it is not a BREAK TEST and STEADFAST cannot help them here. If their leadership has been modified by your Vampire they are likely if you have reduced their leadership to fail it and be reduced to WS1. (Note they can use the generals leadership if he is within 12 inches so you need to consider that, they do not use the leadership of the BSB). Any unit must rely solely on its leadership and perhaps that of the general if he is within range to pass the test. If you remove the general as your first target of priority then he is going to have to rely on his own leadership. If you reduce the leadership of the general himself then your doing fine too.... (Empire General Leadership 9, Skaven General Leadership 7-8* see note below, Dark Elf General leadership 10) If you use enough modifiers then they will have a great chance of failure and most armies that are ok normally on leadership under 8th so far... don't have the magic items points to be worrying about making themselves immune to fear.
Continuing with the above example, if you happen to have a Corpse cart within range, your also going to strike first if you remember to use Miasma of Deathly Vigour. If we happen to have the Helm of Commandment on a nearby unengaged Vampire who might be lurking nearby we have a WS of 6. The humble 50 strong Zombie Horde just became a bit tougher....
Of course their are other ways to do this and as Tim/Steve points out ghouls are just as effective and require less strategy, but it does not mean that Zombies are now useless. I feel that it just requires people to think carefully about how they use units in synergy, and take advantage of fear causing troops and terror causing troops. STEADFAST for all its ability to make your opponent hold after a defeat, cannot help him with a simple fear test. (Note; Skaven with Strength in Numbers, does not apply to my statements, they are the army in which the ranks will help with all leadership tests so be careful if your up against them, but their characters are just as easily removed with well placed spells as well or other items to remove them from play or being effective)
The great thing about Zombies is they are so cheap! If you decide to go down that path, you can have multiple hordes moving forward toward your opponent. Who has two choices; kill you on the approach, or take you down in CC. If he has gone kill on the approach ensure you have compensated, think of what you have to defeat warmachines , think about how to get a unit or two into close combat early (undead summoning might be useful here), think about how to get that pesky damn mage who is dispelling your magic and have a general, bsb plan to kill it fast so that your fear and terror abilities can come into play more effectively.
Hope this helps!