OK, so it's not really Soulstorm's fault that all the cool factions have been modeled and the expansion gets stuck with the Sisters of Battle and Dark Eldar. The Sisters of Battle are somewhere in between the Imperial Guard and the Space Marines not quite as numerous and spammy as the former, but not as tough as the latter. They require a fair bit of micromanagement and, like the IG, aren't a truly potent combat force without the use of vehicles. The one cool thing about them is that they have a new resource, faith, and they consume it to use their special abilities. Balance wise they rank somewhere in the middle, though with their fire fetish, the Sisters of Battle will easily overpower any force comprised of James Hetfield.
The Dark Eldar are essentially a somewhat tougher if slower Eldar race, but again, with the twist that they have a new resource that's consumed to power their special abilities. The ultimate problem with the Dark Eldar and Sisters of Battle is not that they're uninteresting from a lore or gameplay perspective, just that they're too similar to pre-existing races and the diversity is now harming the game. Rather than creating the feeling of more unique options, it seems as if Soulstorm has just blurred the differences among the races.
Where the game really fails however, is in the little things. You can just tell it was rushed. The production quality just isn't there. Where Dark Crusade at least had a series of fitting in-game cutscenes, Soulstorm really doesn't. Dark Crusade would have customized dialog to explain why Imperial Guard are fighting Space Marines, or the different ways the Eldar used to eliminate the Necrons compared to others. The enmity between Chaos and Space Marine was certainly there. Soulstorm has nothing comparable just generic cutscenes for every capital invaded. Despite the presence of three Imperial factions in the game, there's no explanation given why they're fighting against each other.
That's not to say it's all doom and gloom for Dawn of War: Soulstorm. It has improved on Dark Crusade in certain respects. The special provincial powerups are more balanced, so you will no longer be able to attack two times per turn anywhere on the map for example. Still, the provinces retain their strategic significance because of the honor guard units they hold and the webway gates that permit travel between different planets and moons.
Race balance is always a touchy issue in Dawn of War, but that's really not a surprise given how Games Workshop operates. Games Workshop, owners of the 40K license, balance the tabletop wargame so that most of the time, the newest race released or updated to the current edition rules is the most powerful race. This encouraged sales of materials and figurines. Dawn of War has no such incentive but the races are still arguably quite imbalanced. Imperial Guard are just plain difficult to use, while Necrons and Orks are arguably the easiest and most powerful.
The core game is essentially identical to Dawn of War, but given the feature cuts and the fact that this is the third expansion to a game, charging full price for it is rather excessive. Between the lack of significant forward progress and the lower production quality, Soulstorm loses a lot of value relative to Dark Crusade and it's cynical to expect gamers to pay full price.