Including the weaknesses you mention, perhaps Chaos is balanced against Order. Chaos proper is... somewhat limited in that it's pretty parasitic, but it's a real force multiplier and on a metaphysical level impossible to actually stop with any weapon the Forces of Order possess.
Not including those weaknesses? Orcs aren't really 'chaos', but there are a lot of them. Chaos can be incredibly powerful on an individual level, have literally super-magical (it's like supernatural, but even more so) infiltration powers, can do nonsense like 'bring all of their warriors from a thousand year time period to a single army', manifest troops out of thin air, etc. Yeah, they could win, if they really wanted to, but they don't - they want to corrupt, in asmuch the Chaos Gods can be said to have 'wants' in a conventional sense.
The Skaven, though...
The Skaven are a more material race, but they're also a race that has the entire old world utterly outmatched. The only reason they have not destroyed the surface dwellers is they have made the same calculations and concluded - rightly - that other Skaven are a bigger threat. Their population is in the billions, and they have some seriously insane tech. It's all incredibly unreliable and inconsistent, but the highs are modern equivalent. They have ICBMs, for instance, except sometimes they create a hole in reality instead of just a nuclear explosion.
I disagree, a united Skavendom would be able to match and likely overcome almost all surface factions if they were to face them alone but I do not think they would be really a match for them as anything resembling a unified force, hell even the human factions would in my opinion more than enough to beat them if you give them anywhere the same degree of unity you want to give the Skaven. Almost all surface factions perform far better in the battlefield and while the Skaven have their superweapons the rest of the world has their gods and magic which I think easily outmatch the horned rat if not on their own than certainly as a group. I mean the Slann alone have the magic strength to basically reshape continents and there a lot more mages capable of destruction on a massive scale than Skaven superweapons.
No, you're right. Skavens are more numerous than everyone else (except maybe the greenskins) and have machine guns, flamethrowers and combat gas. They could win easily if they weren't constantly backstabbing each other.
It's basically the same in 40k.
I mean I agree that those are factors that weaken the predominat super-power in Wh40k but at least pre universe split I would have argued that said power was the Imperium and not Chaos or any of the other "evil" factions. I mean even with Imperium a moribund, largely stagnated mess for millenia none of the other factions are really capable of opposing it head on if it can concentrate its forces simply due to far larger ressource pool available to the Imperium.