First Point: Yes, you're right, but you're ignoring the fact that in Alloy, Harmony mentioned that the Basin should have heavier-than-air flight at that point, and our own history has proven that war tends to do funny things to people's scientific motivation towards weapons. Also, the destructive capability of Scadrial against cities and large armies (both important fixtures of Roshar) is far greater than vice versa. My theory (and correct this if you disagree) is that even though Rosharan troops will win against Scadrian troops in a pitched battle, Scadrial can use its undeniably superior navy and production capabilities of airships, harmonium bombs, and more warships to simply blast Rosharan cities into submission. In a large scale war, protecting your cities is more vital than winning field battles, especially when their strategy is "draw Radiants to the field, use the Bands/Twinborn/Allomancers/Ferrings/Kandra to occupy them, and then shell/bomb encampments and cities with duraluminum Coinshots or whatever to protect airships.
Yeah, I was off on the timeline. Sorry about that. But you have to admit that Scadrial's technological advancement is much faster than Rosahr's, and they're much further ahead at a similar time period.
Soulcasting isn't that impressive. Here, let me explain. I ran some quick searches and numbers, and Elendel is about the same size if not larger than early 1900s London, (both are around 5 million) which I figured was a good starting point as they're both central cities in large empires that are entering or in the Second Industrial Revolution. (This, of course, doesn't account for the larger world in which unless the Southern Scadrians have massive cities, our earth far outstrips Scadrial in population.) Soulcasting is a crutch for Rosharan cities, allowing them to produce far more food than they should be able to, but it can be disrupted (albeit with a fair amount of difficulty) and despite it the Rosharans don't have a single city close to Elendel, New Seran, or Bilming's size. This leads me to believe that Soulcasting's limits prevent it from approaching the level of industry the Scadrians have.
Another thing that's pretty experimental/theoretical is that Scadrial should have some key technologies at this point. Maybe heavier-than-air is a bit of a stretch at this point, but looking at our own history radio and lighter-than-air are significantly enough to, sequentially, coordinate large navies and armies (both of which Scadrial can quickly raise) and the other has massive destructive capabilities against cities (especially with Harmonium bombs.)
I think your point about the air force is kinda valid. Windrunners are valuable, but they can be blindsided and are at a range disadvantage against protecting lurchers/coinshots, especially ones with duraluminum. I also think that Kandra meatbags in all aspects of the war will negate or at least weaken Radiant advantages. On this, however, I'm very curious in hearing (well, reading) everyone's thoughts.
And the last point, one that might not really be valid, is that Scadrial will have an easier time finding allies in the event of an unusually long war. Cultivation focuses on Roshar, Odium is Cosmere-wide hated, and, as Dalinar and Kaladin are so fond of saying, Honor is dead. Meanwhile, Harmony has tried multiple times to establish contact throughout the Cosmere. Does this count? Yes? No? Kinda? It does, but it's irrelevant? Anyway, all of these are really interesting arguments from y'all.
Wait, one thing I missed, trying to calculate the output of Harmonium is going to be tough because we don't have an accurate control or a varied data set. We have the understanding that blowing up Harmonium is bad, and a battleship worth will destroy Elendel, but (to my memory) the only actual detonation we see is in a subdued environment (a steel+concrete safe box) that causes a fairly large explosion from an unknown (at least exactly) quantity of two metals we have no knowledge of except their the bodies of gods that don't exist in our world, and the explosion itself is a result of a force that doesn't even exist in our universe. Last thing: gravity on Scadrial is different, and the atmosphere/chemical makeup of Scadrial is different. Although you might get some guesses that might be close, the fact that the energy being released by the explosion, the lack of a data set or exact control group and the fact that the surroundings/impacted area might have basic, key differences from Earth means that it's going to be tough to get even a rough estimate of explosive power.
Last thing, I promise. Harmonium I compare to nukes not because of their destructive power but because of how the power is released. Nukes split the atom, releasing a massive amount of energy. It's not *technically* about the size of the bomb, it's the size of the reaction. That's slightly different with Harmonium. Harmonium uses a separation agent, like an anti catalyst (Trellium) in order to...you know what, new sentence. Ignoring Shard's intent and *again, technically* ignoring Investiture, Harmonium will split because Trellium acts as an ignition for a process started when running an electrical current through Harmonium in order to superheat it or get a current going or whatever it is. Trellium's anti-Harmonium nature means that it will naturally try to split from Harmonium, Harmonium is weakened, it splits, and like when atoms split, it releases a large amount of energy. The fact that Brandon, for once, had Wax and company foresee the explosion means that it's deadly. Bad enough, probably, that Wayne putting up a speed bubble and then tapping Gold wouldn't be enough or would be very difficult to pull off. This, of course, is pure speculation. But I think that this is important, because while not having the radioactive property of a nuke or its simpler "detonate, chain reaction, explosion" the Harmonium has stronger explosive capability. The reason why this is relevant is partially because of how Harmony referred to it. An "investiture-energy-matter transfer" or whatever. If you look at basic...physics? Whatever field it is, you see that matter and energy are essentially the same. The ability to interchange these is incredibly powerful. It's, in fact, the basis of life on Earth and throughout the known Cosmere. When you add Investiture to this, you get something that starts closer to fission and ends closer to fusion.
TL,DR or "I can't understand this lunatic's ravings": Harmonium has higher explosive capabilities than nukes but less "let's raise cancer rates than everyone" but potentially can highly invest in an area and potentially rip the Spiritual fabric of an area apart. d
P.S. I wonder what would happen if you detonated one at a Perpendicularity...
P.P.S I'm sorry if some of my connections in that last paragraph are a little hard to follow, I tend to rave a bit and have some trouble expressing how I think things link. Please let me know if something doesn't make sense.
I lied to you all there's one last thing I want to mention. I can cite specific examples if you want them, but quality of troops and battlefield decisions do not necessarily determine wars, especially more modern ones. It's how effective logistics of all kinds and information gathering are, and Scadrial has the edge in both. (Organized government, better abilities at stealth (you know, they're guys don't start glowing when they magic), and arguably most important, radio.) (Also, a big one:troop transportation. Airships like the warship and battleships as well as railways can move larger quantities of troops longer distances faster than the Rosharans can, although the Rosharans will likely win in speed if they have powerful Elsecallers.) So while Radiants and the Rosharan martial tradition will certainly have a large effect, there are other large factors at play in which Scadrial has the advantage.