Orcs are probably the race that has the biggest problems with undead. Even a really crappy UD player can make mass fiends and/or necs, both of which tend to be good counters for most orc units. Tauren with their spirit walkers are of course vastly superior... but that's a tier 3 setup armed with most upgrades and that costs a fortune.

One problem I have as UD is high level melee. Abominations are fairly good, but you can't just pile them up like you can Tauren or Knights. I end up relying on my heroes and summons for melee strength late game. Elves have a similar issue, they have to choose Mountain Giants, Bears, or keep Huntresses. MGs and Bears are not really units you mass, same way that abominations would work better if mixed in with a medium strength melee unit. Ghouls, even frenzied with vampiric aura maxed out, are still paper thin, and Aboms are slow and their best offensive ability tends to be their disease cloud.

As UD, I have problems with Humes, because they have superior casters. The lineup is something like this:

Priest Heal < Statue Heal
Sorceress Polymorph < Banshee Domination Spell
_________________________________________
Priest Dispel > Necro Raise Dead
Priest Buffs (autocast) > Necro Buffs
Sorcereress Slow > Banshee Cripple
Sorceress Invis > Banshee Antimagic Shell

Up against human players I just try to find their army while it's creeping to reduce their overall strength, then move into whatever expansion I've cleared. I fight a war of attrition trying to prevent them from gaining an expansion gold mine while I use my large gold reserves to out gun them.

The nec has cripple, the banshee has curse.

Ah yes. Wrong thread my friend.
And it sounds obscene to say that humans suck balls without specifying that they are indeed bull testicles (Which are considered a delicasy in some parts of the world.). So watch out or you might be misunderstood as a spammer or something of the sort.

Personally, I strongly suggest that you invest in the skills of playing WC3 with Orc instead of Nus. You will find your playing style to be vastly more effective as Nus don't actually exist within the game.

Orcs = Good base fighters/defenders. Easily taken out through ambushes in the field with flying and/or spellcaster creatures.

Undead = Great base defenders and fair field fighters when necros are giving aid. Weak points include becoming to aggressive in attacking bases or no spirit towers.

Night Elf = Good when ambushing, fake attacking, or attempting to win through numbers and/or speed. Weak points are being out numbered, and almost impossible to recoup when unprepared for an attack.

Human = Well rounded, but excells in field fighting unless there are flying creatures. Particuraly weak against surprise Flying and siege attacks.

My bad. Cripple is great, but because it's not autocast it requires extra reserves of micro. Curse is < Slow though.

True, crip is slow to micro, but you should only need to cast it on some heroes, or at least on a few big dudes like frost wyrms.

I could be wrong by the numbers, but in my experience slow on your whole army is a huge pain in the ass. It also probably depends on how good your comp is, because I know from experience that it will affect your micro. On my new comp I win a lot more battles just because I can actually control my army when it looks like all the units are on top of each other.