Concerning casters vs casters, armies tend to be evenly matched in terms of size and strength. Thus, the only deciding factor left is the heroes.
The Orc heroes are terrifying to low HP units. The BM was born to slaughter anything that has low HP, even heroes. The TC's Shockwave is one of the game's nastiest AoE attacks. The FS is equally nasty, except his is fire and forget... Unfortunately. In addition, the Endurance Aura allows them to attack faster. Throw in Bloodlust and you're looking at one crazy army.
Undead has the best AoE spell in Frost Nova. But the sad thing is it isn't working. Why? Mostly because of Healing Wards. The HW's replace the damage quite quickly, and the damage the casters put out is often enough to handle Ghouls or Fiends or Aboms. Stasis Traps are a nightmare to Ghouls, even with WoN backup. The Undead player is often finding himself consuming Healing Scrolls far faster than he has money for. Scrolls of Protection offer little comfort because the heaviest damage taken is always from AoE spells. Going DL/Lich for additional AoE damage capabilities is not an option, as both heroes go down quickly without DK support, and the DK does not offer much in the way of AoE handling, though his abilities at hero sniping cannot be doubted. Without any effective means of protecting their heroes from nukes or healing them immediately after one, the Orc heroes are usually in trouble from him, but their units (unfortunately) do not.
Caster-wise, Orcs have a very effective anti-mass spell in Stasis Trap. Most Undead players are smart enough to keep a WoN or two (preferably the latter) on them at all times, to remove the annoying Traps, but the charges are often entirely spent after a few seconds of battle, what with BL, Lightning Shields, HWs and Stasis Traps flying around, unless the UD player is like me and is being extremely miserly with his WoN charges. Plus, their DPS goes nuts with BL, and they can easily heal damage from other casters with HW.
Undead casters are pretty strong, but the Necros' Skellies are more often than not a joke to the Orcs, even more so than the Ghouls. With level 3 Shockwave, the Orcs no longer require WoNs to handle Skellies, as a single Shockwave would knock over an entire legion, and granting huge experience to the heroes. Curse could be an option (Curse is highly underestimated by the Warcraft 3 community), but maybe not AMS. Cripple on the heroes is a possible, and maybe even on some important units. Unholy Frenzy works when used on Fiends, as it sends their DPS off the charts, but the Fiends themselves go downhill past late-mid game. Possessing Taurens is one thing, but Possessing casters is different. Capturing Witch Doctors might help, as the Healing Wards and Stasis Traps could be nasty surprises for an Orc player, but hasn't happened for me very often. One of the few times when I curse the 1-second delay for Possession.