The undead are "supposed" to lose troops though, what with the necromancy shtick.
I know, but offsetting the disadvantage of unit losses incurred is fairly important at several levels of play, and the Undead somewhat need to take advantage of degenerate usage of their corpse use. Making a number of extra Graveyards to provide for Cannibalize, using Meat Wagons purely to preserve and transport bodies instead of their intended siege role, massing Necromancers and Obsidian Statues to generate large groups of disposable units and making Banshees to steal enemy units
primarily to cover the Undead's own weakness in unit value by either stealing a healer or snagging a beefy unit to run cost-positive with the Banshee in question with a good complexity to value ratio.
The corpse usage is a
problem for lower-skill players, as it traps them into FOO strategies that don't scale upwards without unreasonable jumps in skill (such as massed Meat Wagons and Necromancers with Obsidian Statues sprinkled in, which needs immense micromanagement to pull off, or yet more build order complexity to add Crypt Fiends or Gargoyles). Adding a new corpse generator, or a proper combat healer, helps with the lower end of play to form better habits as you go up the ladder. This is the
exact issue with the campaign, as it doesn't actually present the gameplay incentives that you need for PvP success, leading to segregation in gameplay. It's such a severe issue that Blizzard flat out
gave up and made co-op in Starcraft 2 to offer the campaign's incentive structure in a multiplayer setting. This is a
big problem for the bar to entry into competitive play.
Actually, speaking of the bad incentives, the actual campaign missions could be used as a true tutorial to the game, giving you scenarios where you have to enact skills needed for competitive play. Higher difficulties could do more than merely alter resources and unit stats on the enemy to instead offer actual changes to test higher-level skill, such as adjusting enemy behavior to force you to micromanage two or three combat fronts at once, a skill that has proven extremely important at the highest level of competitive play because it allows for pressuring an opponent's multitasking skill, something that few people are particularly good at.
Instead of the ubiquitous defense missions of holding out until reinforcements arrive, the goal should be to destroy the enemy before
their reinforcements show up so that you can accomplish the in-story objective without much opposition. "Defense" actions in-story could transition to actually breaking down the enemy's army instead of just holding the line, working to
eliminate the threat directly instead of just preserve resources. The only case where a "hold the line" mission should
happen is if a PvP viable turtle strategy is intended, so that the mission's lesson on how to handle a situation carries over directly to PvP play. Even then, a limit to losses or resources should be included so that the needed skills to do it are enforced by the mission.
