Damn, I registered on this forum, when found that topic (accidentally), only to add a reply on it 😄.
Well, I must admit I am a StarCraft fan, and I play it almost since it was released. My knowledge about WH40k universe is much lesser, but still quite considerable.
I would say that StarCraft races would probably lose, but this result is not so certain, and even if they would've lost, it wouldn't be a slaughter. Many WH40k fans underestimate SC races (often showing, that they know little about them), while overestimating those of WH40k universe.
On the start, that this comparision is a nonsense from the start. Any comparision between two universes, created for different purpose, by different people, basing on completely different ideas, is a nonsense.
Firstly, take a look at the timeline of those two universes. The first one takes place after year 40 000 - it is obvious, that all races should be powerful and have many planets under their influence. And Starcraft? It's plot starts basically in... 2499 (destruction of Chau Sara). In such "young" age, those races are already powerful - try to imagine, what powerful would they be after over 30 000 years of development? Seeing, that they are already introducing new technologies after only a few years of war (new units and abilities), I suspect, that they would be much stronger, than all WH40k races alltogether. But, of course, I'm only guessing.
Another thing is notoric overestimating of WH40k races, and telling about all those technical miracles, that appear to be just a big bullshit, when saw in the battle (table top or computer games), or when just analyzed. Space Marines' power armour. A magnificent piece of technology, that can resist most weapons... and yet it is easily penetrated by a primitive axe or cleaver, yielded by Orcs. Like I said - bullshit. Not to mention the Terminator armour, that is always described to be "virtually invulnerable", and yet it is easily penetrated by... claws (Genestealers). Man, is that a joke?
Bolters. Basically rocket-launchers, that (as someone said) will turn unarmoured human into a mist of blood and guts. Somehow many Imperium's enemies take a little or no armour at all into battle (low-level Orcs, Kroot, Vespid, certain Chaos units like Cultists, etc.), and they survive that. When you play Dawn of War, you can see, that even a burst of Bolter projectiles is unable to kill single Kroot, that does not only wear any armour, but is even naked. Another joke?
Imperial Navy's weaponry. Someone described it, that weakest projectile of it has a hypernuclear firepower (BTW what class of weapon do they have? Fusion cannons? Plasma? Beam cannons?). That's really shocking, but... where is that huge firepower, when I see the arts, movies, battles in the computer games? Any ship from WH40k universe, that is of an equal size, can engage Imperium battlecruiser, and fight for quite long. Then I'm asking, how is it, that all Imperium's vessels' guns have firepower bigger than an a-bomb, and yet they cannot destroy another ship with a few shots? Take that cinematics from Fire Warrior (poor game, ofc, but is stilled based on universe created by guys from GW, and signed by them), where a battle between Tau ship and Imperium ship lasted for few hours. If humans of the future would have such firepower, everything would take minutes, at most.
Fairy tale about Space Marine swallowing a grenade is so stupid, that I don't have a nerve to comment it. THAT is a very poor joke. Why someone won't tell me about an SM flying in the air like Superman? God, help me...
What is the basic problem with the "realism" of WH40k universe? That they make up so goddamn-bloody-unbelievably-incredibly-powerful technological miracles, and then forget about them, once we launch Dawn of War or decide to play on table top. What's the point? Of course, I listen to all those lies, but I make a silent contract between me and guys from GW. They say "let us make up incredibly powerful weapons for races in our universe, so that all this war stuff would look really astonishing, and try to believe it". And I agree. I believe. I believe, that bolters can turn human being into a bloody mist. I believe, that guns of Gothic class battlecruisers are powerful enough to destroy an entire planet. I believe all this, and I don't argue, although I see, that those are lies, when I play games released by Games Workshop. I tolerate it, because all this stuff is great anyways.
But I won't tolerate using all those lies as arguments in such discussion. Especially when WH40k is compared to the universe of the game, that I love most.
And about StarCraft races... in this case, many arguments of WH40k fans made me laugh.
Psykers. Everybody keep telling me, how tough they are, that they could simply fry Protoss' brains and see, where Ghosts are (and fry their brains too). In either case, it is a total bullshit. A human psyker, frying mind of a PROTOSS? On the one side we have a person, that is gifted and powerful, but still is just a primitive human, and on the other side an effect of millions of years of evolution, far superior to human, that does not only have psionic powers... but, in fact, is a being, that is MADE by psionic power? And you try to tell me, that this primitive human, which is but an ignorant is case of psionics, would simply fry a bunch of beings far superior to him? Yeah. Sure. Another joke.
Next joke is connected with Ghosts. Psykers may try, but they won't feel them. Detecting Ghosts by psionic powers is impossible. That is because they are implanted with psionic inhibitors, that "lock" the access to their minds, and make them invulnerable to the attempts of "feeling" them or reading their thoughts.
Someone mentioned, that void is the most powerful force in the universe... did anyone know, that void, is a force, that is used by Dark Templars from StarCraft? Thanks for giving me an argument!
Imperium of Man, that "will win, because its population is quadrilions of quadrilions of people". And I just ask: SO-DAMN-WHAT? How many of those quadrilions are Space Marines? Wait, I remember - one milion. One milion! Another joke today! All, that Imperium can dispatch, is one million of soldiers in power armours, while Terran Dominion itself has a few millions of soldiers with such equipment (bah - EACH Terran soldier has power armour!), along with hypersonic rifles, powerful enough to penetrate the Adeptus Astartes power armour. I even don't count Imperial Guard, because these guys are ridiculous with their bulletproof vests. They would be slaughtered by Terran infantry - a SINGLE shot from Gauss rifle in the stomach, and Guardsman is down. Basilisks, Hellhounds, Baneblades, and other vehicles? Terrans have their Siege Tanks, that are beasts. Not to mention the Ghosts. One lockdown on Baneblade - and it becomes just a target for practice shooting. And there are people here, so naive, that IG would take down all SC races. On their own, they wouldn't be able to endanger even single Dominion.
Another thing, is that you only compare the firepower of the races. The weapons from StarCraft are weaker, of course. But races from SC also possess many special abilities, that would nullify the firepower of the Imperium and others. Psykers? Bring in a Science Vessel, launch EMP shockwave, and guys are useless. Predator tanks? Dreadnoughts? Bring in Ghosts and lock them down - and they are worth shit, just like Psykers. They would be helpless, taken down one by one. Imperium battlecruisers' firepower? Use Defensive Matrix, and Behemoth class battlecruiser would have enough time to use its Yamato cannon, and bring the enemy down. Bunch of Space Marines? Single Psionic Storm, and they are all fried within a seconds. I can give a lot of such examples. A fleet of Imperial battleships? Use Arbiter, cloak all Scouts and Carriers, and they are helpless. A few special abilities from StarCraft, and brute strenght of WH40k races is worth shit.
Another thing, is that all three races' arsenal gives them quite considerable chances in the fight. Behemoth class Battlecruisers with their Yamato guns. Yes, they can't destroy an entire planet with it. And I ask once again: SO-DAMN-WHAT? The firepower of Yamato gun is huge enough to destroy entire battleships with single shots. Even Protoss vessels, protected by shields, are brought down by them. What chances do Gothic class battlecruisers, not protected by shields, have? If only Terrans would have the advantage in numbers - e.g. there are 12 Imperium cruisers, while Terrans have 15 Battlecruisers - the Imperium fleet is in big trouble, they would be destroyed one by one by Yamato gun shots. Defensive Matrix, mentioned earlier, would give Behemoths time to load these guns - and then, all Imperium vessels are space dust. Even their "hypernuclear projectiles" won't help them.
What? That Terrans don't have many ships? Look at the UED campaign outro and see that fleet above Char. It is just one third of an expeditionary force sent to Koprulu. And that whole expeditionary fleet was an ANT, compared to all UED's power. And UED is not the only one Terran faction in SC.
Zerg vs. Tyranids. Yeah, Tyranids are greater in numbers, but that doesn't mean, that Zerg wouldn't be able to overwhelm them in a single battle. When Psi Emmiters were used on Antiga Prime, and Tarsonis, there were enough Zerg to cover the both planets, like bloody carpet. WHOLE. And to fill the entire orbital space. And yet, it was only a small part of an entire Zerg Swarm force.
Another thing is that the Tyranids wander in space, and thus their recources are a bit limited. Zerg, on the other hand, set hatcheries on the planets, and are breeding new soldiers fast. It could appear, that Tyranids, when they would try to invade Char, for example, they would deal casualties to the Zerg slower, than the Zerg could regenerate them.