Disagree. They have nothing empowering them to be strong early game compared to other cultures.
Their economy is non-existent. Their units are of average power and their combat passive is weak.
I'd take a 1v1 with any other culture vs Dark and guarantee I'll win every time. They have no benefits.
They have very good units, in the early game, because they have access to two shock units, which means they can wreck basic infantry formations. In the late-game, everyone has powerful units from their tomes, so base dark culture simply fails to adequately fill in the 'gaps' in their tomes and rather they have to research tomes to 'replace' their regular units.
When playing versus the AI, sure. That's the only time "horde tactics" can potentially work. This is absolutely not the case in multiplayer.
Fights are so unbalanced that generally speaking they end 18v3, it is extremely rare for a fight to go somewhat even between players.
And this is when both players bring the same level of units. If they bring weak "chaff" then they will get absolutely wiped every time.
Example from yesterday (spoiler, Wyverns still suck). The player piloting the Wyverns is the best player I know of.
He even tried to "abuse" Draconic Rage by attacking his own units and then healing them back to full HP after.
Also the player on the left received Spider Mounts and Demon Step from the shop. Winning thanks to RNG imbalance.
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When I played against the AI, I ended up with only four units left from my second, victorious wave, so it wasn't exactly cheap. The main problem I faced was that the AI cheats meant they could quickly replace their doomstacks in the middle of a besieged city, but none of that applies to another human player. You cannot win a war of attition against the AI, because the AI cheats.
If you don't turn off simultaneous turns, (they should actually just abolish it from the game altogether), then yes the doomstack problem cannot be solved in multiplayer. Without classic turns on however, players cannot avoid their doomstacks being swarmed since they cannot move damaged doomstacks away in the other players turn.
The wyvern-loving player you mentioned, they killed about half the enemy army, in particular they killed three ironclads, two heroes and damaged the remaining two ironclads. They should have retreated with their heroes though, rather than sacrificing them like that, that way they could created a new stack of wyverns for them to lead rather than having to spend gold on buying new heroes, which could have gone on wyverns. A good build here for swarming would be to make sure to get Order Adept for your heroes, so you can buy Keeper's Mark, allowing them to escape death more easily, especially if they are mounted or wearing speed-shoes.
If a second wyvern stack were deployed at this point, they would either win, or the opposing player would be forced to retreat after so 'pyrrhic' a victory, allowing the other player to quickly replace their lost units, since they no longer have to pay the upkeep of their 'dead' units. Given they are both human players, neither gets to cheat their way to a new army, meaning that a war of attrition could actually work.