- по Heroes 3

You are wrong; on the 3rd day, you can build a wyvern, and that results in a quite powerful army! The Swamp is a strong town; it just has its own tactics, which differ from the classic tactics used in all other towns!

There is no classic tactic. And the Swamp is a weak town; you can only build a wyvern on the third day under normal game conditions. Under the same conditions, other castles on small maps (which is where this strategy is most often used) can build quite decent counters for 2 wyverns (especially since 2 unupgraded wyverns can't do that much).