What about siege engines? You don't necessarily need Sulfur Rain, but the catapult perk will be quite sufficient. The remaining free slots can be used for a ballista and a rapid-fire ballista, which will give the Orcs some ranged advantage. Moreover, in online play, a ballista is always in demand. Well, let's say the catapult doesn't destroy the gate on the first try, but it might on the second, and in that time, you can bring Executioners and Shamans to the gate. So, reinforcements for the Wyverns can arrive quite quickly.

What can I say - it's an isolated case! ... specifically, defeating the dumb AI in the Arcanum campaigns, where you can do so much, and time things so perfectly, just by using a single save/load.
As far as I remember, some missions were completed, for example, using only Goblins, or, like Ornela, killing everything in her path with about 200-300 Vampires, while the enemy armies were 10-50 times larger than hers.

Here, I disagree. It depends on the difficulty level. If you play on Hero, the campaign can take a long time and create a lot of problems.
For example, here's a replay of clearing the guards at the gold mine. Before the battle, the threat from the neutrals was assessed as very high.