*flexes Geek Muscles*
Any single Universe in which any Fiction is played out has its own Rules and its own "Overpowered" Race so to speak.
In Dragonball, or rather Dragonball Z the actual End was supposed to be the Freeza Saga, therefore Son Goku was supposed to be the only Super Sayjan to exist. Not Vegeta, not Gohan, not Trunks etc pp. That was added later on because the Series didnt stop where it should have, so the Enemies got stronger and they added more and more "Forms" of Super Sayjans, like 2, 3 and 4, Ultra etc. At some Point it just got ridiculous.
In Warhammer 40K, all Races, despite their own Drawbacks and Advantages are fairly equal. Games Workshop will of course keep it that Way. They do favour Space Marines of course, which is very easy to see. But they are not going to make one Race powerful enough to wipe out the others.
For example, if the Chaos Gods would all combine their Powers, and equally all the Traitor Legions would once again form a Crusade which would rival the original Horus Heresy (No the current 13 Crusades havent reached that) Then yes, all other Races are basicly done for. Because the Emperor could not stop another Horus, let alone one of the demonic Primarchs. So if that were to happen, the Imperium falls.
If the Tyranids would attack in Force, yes their current 3 Hive Fleets are impressive and powerful, they are NOT their entire Force. By default the Tyranids are either fleeing some other, even more terrible Enemy or are drawn by the light of the Astronomican, which is powered by the Emperor's own Psychic Powers. Again if they were to show up in full Force, then you can kiss your Ass goodbye.
The same goes for Necrons, if all the Tomb Worlds across the Galaxy would rise at the exact same Point and attack in full Force every living Species, they would at the very least wipe out a few of them. However they are driven by the C'Than, and while only 4 are fully known to exist, each is planning something different. If of course they would focus on a common Enemy, again, good night at that point.
Every other Species has its Turning Point that, if they would achieve it, they would instantly win the entire War. The Imperium just needs to get back its full Technology from the Dark Age of Technology, if they achieved that Level again, or got very close to it, no other Species has a Chance, because at the height of its Power, the Imperium could hold all other Forces at Bay without even trying. This includes the Orks AND Eldar, who back then were still a giant Empire of Worlds. The Eldar on the other Hand are trying to "breed" a new God, if they succeed instant Advantage over everyone else.
So yeah, no Race is clearly superior unless those Points are met, because GW wants to keep 40K alive and People playing it.
The same goes for every other Species in other Stories/Movies/Shows, while having a gigantic advantage, they are not powerful enough to stop other Races in their Tracks, or if they are, then they simply do not care, because at that Point, no other Species is a Threat to them anymore.