I think it's more common in traditional fantasy for the heroes to be underdogs fighting against an enemy who outmatches them and who they have to pull out all the stops to defeat, rather than one whom they already outmatch.
If anything though, I would say that Warhammer Fantasy is a rare example of the reverse. The Empire in particular are not the underdogs, they're very much in the ascendant. They're the status quo everyone else is trying to undo. They're also good at everything. They have actual universities to train powerful wizards. They have advanced technology and artillery like the dwarves. They have amazing knights and, more importantly, they have huge numbers of all these things. Other factions may excel at one thing, but the empire excels at everything.
Chaos in Warhammer fantasy are a bunch of tribal barbarians who live in the frozen tundra, smelly goat people who hide in the forests and harass peasants and daemons who can only do anything under the right weather conditions. They are the underdogs almost all of the time. The fact that they constantly fight each other just makes everything worse.
Once every few generations, chaos becomes a threat. Maybe there's a big storm and daemons can start doing things, maybe there's a powerful warlord who unites the various tribes under one banner. At these moments chaos becomes an existential threat to the world. Warhammer: Total War takes place during one of these times (canonically, the one in which Chaos actually succeeded). But it's important to remember that even taking into account these big events, the Empire's win loss ratio is 12 to 1. That's how dominant the Empire is.
Heck, the reason Warhammer 3 is so focused on chaos is because chaos currently really sucks to play, because they're done in a lore-friendly way that completely gimps them. They don't hold settlements or territory (excpet for Norsca, and even then they're very limited). They can't because their Gods are useless outside of the realm of chaos. The only thing they can do is try and burn the world down, and inevitably get squished by better organised factions who can actually hold the territory they conquer.
Okay, taking that as writ, I can't really argue against that, but I'd counter with the facts that:
-Something that's repeatedly emphasized in Warhammer is that life is terrible for pretty much everyone (least if you're a human in any Old World kingdom - maybe you'd get a better deal in the East), and that on a global scale, things are bad, and will never get better. Chaos can never be defeated, and many times, it's written that people are living in the twilight period of the world.
-You can argue that the Empire is doing okay, but if we look at the 'good' races, how are they faring? Well, the High Elves and Dwarfs are in decline. The Wood Elves are content to stay in the forest and do nothing else. The Lizardmen have lost the first generation of Slaan, so they're kind of operating blind, and aren't really a world power. Meanwhile, greenskins are still a threat, the skaven are still a threat, the undead are still a threat, etc.
-On the subject of Chaos, putting aside End Times for a moment, yes, you're right, Chaos has always been repelled in the end, but it's established that the Chaos incursions are becoming more and more frequent by the 'present' of the setting. For instance, 200 years pass between The Great War Against Chaos, and Achaon's assault. That's a shorter gap than prior incursions. So, with the incursions getting more frequent, and more powerful, quoting from 6th edition, there may come a time where it goes from 200 years, to 100 years, to 50 years, to every year, to, eventually, a never-ending tide. The Empire can repel Chaos every time, but Chaos only has to win once.
As Terminal Blue said, Warhammer is pretty much the opposite. Which is why I like the Warhammer lore quite a bit. The forces of Chaos are weak and disjointed, mostly being relegated to working in the dark recesses of the Empire or on the fringes of the civilized world that are almost uninhabitable to anyone else. Not only is the human Empire strong, so are the High Elves and the Lizardmen, the latter of whom is the old servants of Chaos ancient nemesis. Chaos can occasionally get its stuff together and send out a massive attempt to burn the world down, but just as they are slapped back down by the forces of order and things return to normal.
For all its bleakness and darkness, the world of Warhammer Fantasy is an unusually optimistic take on Dark Fantasy. The Humans are explicitly winning most of the time and while it has touches of dung ages, it also portrays humans are genuinely progressive and intelligent.
I'll grant you that the Empire is strong, but the High Elves and Lizardmen? Really? The High Elves are explicitly stated to be in decline, and the Lizardmen aren't really a world power. They can only guess what the Old Ones wanted for the world, and what the Old Ones wanted for the world probably isn't what's good for the people currently living there (how many people will die if you shift a continent for instance?)
As for humans being intelligent...really? The Norscans are tribal barbarians, but if we confine this to the 'good' humans, then what do we get? Bretonnia may be the land of chivalry and whatnot, but it's still a feudal society where if you're not a knight, your life is terrible. The Empire may be more progressive, but it's got no shortage of superstition within it. I remember in flavour text for Middenheim that a scholar suggested that the comet which destroyed the titular city was a natural event, and that he recanted his opinion before being castrated by his peers or something. That, in addition to flagellants preaching the end of the world, and Chaos cults, and all that, and yes, the Empire may be more enlightened than every other Old World kingdom, but that's a pretty low bar.
Maybe Cathway, Ind, and Nippon are more enlightened, but that's hard to say, since they've been explored far less.
