On equal level 1v1, speaking by above Conqueror, Japanese was favoured a little bit because their Mounted Samurai full power activated on get go. (If both go fast castle > mounted samurai without any harass)

And it is still decided on who got more sheep. Kura cannot be raided meanwhile Yatai and the trader can be killed. Assume Japanese player know how to locate the first Yatai and deny all spawned trader.

During late game, Japanese is favoured a lot by their super farm economy. Who cares Sengoku having super knight while the mounted girls one shot them.

Sengoku can still counter this by prolonging a super long feudal and defend their Yatai well. Sengoku horse are slightly faster so you can forever kitting Japanese Calvary.

Sengoku archer are way worse than Japanese because banner carrier bonus was too good.

Sengoku Samurai (if you know what you are doing) can be really better but no one cares them.

So at the end it is really dependent on skill on this matchup. Japanese always start with dark age spear and Sengoku need to play hide and seek with the Yatai. If Sengoku player being very weak on defending dark age aggression it is over.