The Horde does have characters of extreme potency, they just don’t ever get used in the narrative, or have been retired.
Thrall’s gone off to do his spiritual journey thing, though in Cataclysm he became an extreme shaman savant, rather than what he normally was.
Drek’thar is old and frail, but he’s still a farseer. Being able to follow an enemy should be a huge boon.
Grand Magister Rommath is a man with many years and many lessons under his belt. He just doesn’t get used. Him being able to fling out fire storms and summons phoenixes on a whim would even the odds against any magical character.
First Arcanist Thalyssra is also a huge and prominent magi. She’s been doing this for centuries. She doesn’t get used either. There’s also those under her employ who are exceptional in their work, such as Occuleth and Vlatrois.
Nathanos, while not magical, is a marksman and a skilled woodsman. If Warcraft applied to the rule of one, he’d be a capable assassin for any would-be powerhouses, like Malfurion, or Jaina.
Sylvanas, while currently a Warchief who is soon to turn into a raid boss, is a banshee with the ability to raise the dead. She’s also an experienced general.
As for the Horde, it has plenty of units to even the odds on the battlefield, they just aren’t used either.
The core races have shamanism and have been entrenched in it for longer than any of the newer Alliance races have. It’s familiar territory for them.
The core races also have wyvern riders, venomous winged monsters. They’re a good equalizer to gryphons and hippogryphs.
The Forsaken, of course, has the blight, an airborn poison. Chemical warfare is a no-no in the modern day for how lethal and indiscriminate it is. Just the same, the Forsaken used to be members of the living themselves, it’s fairly plausible that them and the Alliance still have the same understanding of formation battles.
The Blood Elves, while not a heavily populated race, are masters of the arcane.
The Tauren can excel in just about everything they want to do. They’re massive bull men that’re 12 feet tall and packed to the brim with muscle. They could carry long lances and outreach any enemy on the field. Their habit of using those battle totems as a bludgeon is also a boon, as it can overcome any man-held defense, no matter how much they hunker down into that shield. The implications of their physique make them a wonder.
The Goblins make technologies to equalize with the Gnomes & Dwarves. Their technology is by no means pretty and looks as if it were prone to an explode, but function over fashion.
Trolls are also tall, long armed and would make exceptional pikemen.
This isn’t to say the Alliance is inferior by any means, it’s moreso the advantages the Horde has on their side that stops the Alliance from bowling them over in any field fight.