Barbarian magic suppression skills...

So, let's break it down: lords with 2000 hit points are certainly a formidable opponent, but I don't see anything catastrophic. With that many, Horde Fury will quickly deplete their hit points (by the way, what about Phoenixes? Will one of them survive a single hit?).

The Shield will last for about 4 weeks, but how much damage will Horde Fury inflict? And we shouldn't forget that killing goblins and centaurs isn't a particularly long process. After that, you can basically ignore Horde Fury. Things only get worse from there, because the Necromancer will definitely have more than one growth per week thanks to the Converter. 40 lords per week is more than realistic.

And, of course, we'll be targeting vampires first (preferably before the hero's turn).

It's very likely that you won't even reach them with your first move, because having 2 artifacts from the racial set is quite common. And even one Rise Dead spell doesn't reduce the effectiveness of vampires; sometimes, it even increases it.

Next, mines: 250-70%=75, that's not serious, a wasted hero turn.

With a single move, you collect far more than one mine, and you'll take damage and have your morale reduced. Let's say you hit 4 mines, absorbing 700 damage. For example, a stack of 100 Warmongers will have 1100 * (700/2000) = 385 morale deducted. They can't recover from that, and they'll be killed immediately.

Now, imagine for a moment what the opponent can do with Darkness in just a couple of turns, even with it being heavily suppressed!

I agree that Darkness is usually more effective, but what if you have a set of spells? And if not? We look for other options and find them.