Game Breaker: The stealth-capable Fremen soldiers in Emperor can be this if used right. In the final mission, you can easily bypass the entire battle by training a group of Fremen, manually moving them on foot straight through the enemy base (without attacking anything), then have them attack the mission goal structure, move them out of the way of the slow-firing defensive cannon, and repeat until it's destroyed.
House Ix has invisible, fast-moving infiltrators that explode in a suicide move that is devastating to buildings. A clever player can easily navigate a number of them past a base's defensive line and blow up any number of critical structures. Ixian tanks can also create functional hard light copies of any other land unit which, while killed in one hit, retain all the destructive power of the real versions. Five or six of these tanks can easily clear entire maps simply by spam-copying certain units such as the Ordos laser tank and throwing them at the enemy.
House Ordos laser tanks can fire on the move and outrun any unit capable of beating them in a one-on-one fight. Their powerful laser turrets also rip through enemy vehicles and structures, making them terrifyingly effective at hit-and-run tactics. They also have both shields and regenerative health.
Although an absolute pain to micromanage due to their tendencies to cause friendly fire incidents, sonic tanks have both great range and damage output. A line of these tanks can sweep a map clear from one end to the other if kept repaired and appropriately supported.
Harkonnen anti-air drones are equipped with quad-barreled machine-guns that can, for some inexplicable reason, target ground units in addition to air units. Although slow, their firepower is immense, and only a few dedicated anti-air units and structures can even target them.
Minotaurus walkers fire long-range rocket barrages that are devastating to both infantry and armor. While they can't fire whilst moving, they are essentially the ultimate base-defense unit. Their only weakness are air units, but couple of Mongoose thrown in can counter that.
Special Effect Failure: The FMV's suffer from this. The actors' skin will occasionally have green highlights when there is no green light source nearby. The imperial crown is also very clearly made of rubber, and characters may occasionally stretch it out when placing in on their heads. A not-quite knife fight between Gunseng and Copec features an obviously fake knife.