The Undead have a special gold structure

The Undead have a special gold structure -- a Haunted Gold Mine. Much like Night Elf Wisps, Undead Acolytes remain on the mine and TBC Classic Gold collect automatically. In 100 seconds, building one is slower than the Entangled Gold Mine, however, Undead Acolytes don't have to stick around in one place to construct, instead summoning a portal that attracts their constructions into existence. When the portal is summoned, they're free to start other jobs.

Undead also differ in the other factions in that they need to use their most basic combat unit -- Ghouls -- to gather lumber. The trade-off is that Ghouls gather 20 lumber per delivery instead of the Individual and Orc's 10, along with the Night Elves' 5. They cheap WOW Classic Burning Crusade Gold are also much more defensible, if your competitor target your workers.