Disclaimer: This is an area still debated between traditionalist and revisionist. I lean revisionist (though not radically). So take what I say with a pinch of salt.

Part 1: The Horses

So this is what Jesuit Luis Frois say about how European horses and horse-riding compare to their European counterparts:

1. Our horses are very beautiful; Japanese horses are greatly inferior to ours.

2. Ours can be brought from running to a halt in quick order; theirs are very hard to control.
3. Ours allow riders on their backs; Japanese horses are not accustomed to this.
4. Ours are accustomed to going along side by side; those in Japan always follow one another.
5. We leave the tail of our horses loose for beauty’s sake; theirs are bound in a knot.
6. The longer the mane on our horses, the prettier they are; in Japan they cut the mane and at intervals they attach pieces of wheat straw, to enhance the horse’s magnificence.
7. Our horses are all shod with iron horseshoes and nails; in Japan no horses are thus shod, but instead are fitted with straw shoes that last for a couple of miles.
8. Among us, the footman walks ahead of the horse, leading it by the halter; in Japan, depending on the condition of the road, the footman is loaded down with straw shoes for the horses.
9. Among us, the bridle has a little tongue and rings that go inside the horse’s mouth; in Japan they have nothing more than a piece of iron crossing the horse’s mouth.
10. We mount with the left foot; the Japanese with the right.
11. We use a saddle and full-length stirrups; in Japan they ride with only short stirrups.
12. Our stirrups are made of iron and are open in the front; theirs are made of wood, closed in the front, and very long and narrow, like Moorish slippers.
13. We use spurs; they do not, using instead only very short stalks of vara, which are like our reed canes.
14. The pommel on our saddles is completely closed in front; theirs has a hole that one can grab and hold onto.
15. On our horses we use cruppers, caparisons and equipment adorned with brass tacks, 18 on the horses in Japan they use none of these things, only a type of caparison made of tiger hide with the fur side out.
16. Our saddles are made of leather and wool; theirs are made of wood and lacquer.
17. Our stables are always behind or in the lower part of the house; in Japan they are built at the front of the house.
18. In the homes of European nobility guests are first welcomed in the living quarters; in Japan their first reception takes place in the stables.
19. Our horses are cleaned with a currycomb; theirs are cleaned using either the hand or a tool made of cords.
20. Our horses have mangers; Japanese horses eat from low troughs.
21. In the stables of our nobles, the horses often lie down; those in Japan, day or night, are nearly always kept standing by a belt tied around the belly.
22. Our stables have earthen floors; theirs have wood plank flooring.
23. Horses in Europe urinate on the floor of the stable; in Japan they remove the horses’ urine with long-handled ladles.
24. Among us there are mules, zebras, donkeys and pack animals; there are none of these in Japan.
25. Among us, only mules wear a long saddlecloth; the horses of the Japanese nobles wear both round leather saddlecloths and others made of straw.
26. For us it would be ridiculous for a nobleman to go about with the halter on his horse and the lead rope in his own hand; in the Kingdom of Bungo, the sons of the king often go about in this manner.
27. When we gallop or ride on horseback, the reins are held in one hand only; in Japan they are held in both hands.
28. Among us, bleeding is the only treatment used on horses; in Japan, while they are bled often, they also place large cauterizing irons under the jawbone.
29. In Europe the reins are loosened to make the horse run and tightened to make it halt; in Japan they are loosened to stop and tightened to run.
30. We till the earth only with oxen; in Japan they use either horses or oxen.
31. Packsaddles in Europe are made of cloth and straw; in Japan they are made of wood.
32. Among us, no load is carried without a crupper; in Echizen these are not used.
33. Our pack horses wear bells and rattles; in Japan they wear metal disks like those on a tambourine.
34. Our bulls are huge and mean; in Japan they are small and tame.
35. In Europe, muleteers burden their beasts and carry nothing themselves; in Japan, out of sympathy for the beasts, the muleteers sometimes carry a third of the load on their own backs.
36. In Europe, the load to be placed on a beast is determined visually; in many kingdoms of Japan, nothing is loaded on a beast that is not weighed first.
37. Among us, an unsaddled horse is led by a man using a halter; in Japan the horses of noblemen, even those that are very gentle, have to be moved by one man with a rope in front and another with a rope in the back, like a roped bull.
38. Our saddles are strapped onto the horse using a girth beneath the saddle; Japanese saddles are secured with a strap over the front pommel.

The low quality of Japanese horses was something often repeated by European observers.

Not all of that is relevant to our discussion. But some are:

Japanese horses are hard to control. One of the reasons is that horses were not gelded (castrated). So having stallions with other stallions plus mares in heat are big problems. Japanese seem to have preferred a bit of wildness in their horses, but it likely caused problems. The Zōhyō Monogatari spends many words telling grooms how to prepare the horses for battle, and what the consequences are if they don’t.

Japanese horses were smaller and weaker. NHK tested pony of roughly the size of Japanese horses back then carrying the equivalent weight of a fully armored men, and found it could go no more than 9 km/h. It could only canter at best, and often dropped to a trot. This is matched by the rarity of the word 疾駆 (gallop) in written sources. At least in leisure, Japanese elites seemed to prefer to take it slow when riding anyway, leading Townsend Harris to say “The Japanese are no horsemen”.

The lack of iron horseshoe necessitated the constant switching straw wraps, which will impact how far a horse can go on campaign.

And, very importantly, due to a lack of other pack-animals, horses (and men) need to take their places in plowing and transportation. This means a horse available just for the samurai to ride, whether as leisure or into combat, is rarer than it would be in Europe. Such a horse would be as much a status symbol as an asset in war.

The Japanese horse does have two things going for them. One is that they seem to have been quite stable when moving. The other is that they seem to have adapted to moving around in hilly terrain more than other breeds.

Still, we should consider the Japanese horse unsuited to delivering a heavy charge, something cavalry did in the Eurasian mainland all the time. And while I do not know the primary source for it, I’ve read multiple times that during the Korean expeditions, the Koreans and Chinese thought they had the advantage in cavalry.

A Japanese horse is fine when the samurai cavalry were horse archers shooting at each other before riding in to engage in one clash with swords. But what happens when that’s no longer effective?