The Undead have a particular gold structure -- a Haunted Gold Mine. Much like  TBC Classic Gold Night Elf Wisps, Undead Acolytes remain about the mine and gather automatically. At 100 seconds, building one is slower compared to the Entangled Gold Mine, but Undead Acolytes don't need to stay around in one place to build, rather summoning a portal that attracts their structures into existence. Once 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 simplest combat unit -- Ghouls -- to collect lumber. The  cheap Burning Crusade Classic Gold trade-off is that Ghouls gather 20 timber per delivery instead of the Individual and Orc's 10, and the Night Elves' 5. They're also a lot more defensible, should your opponent target your workers.