i listened to the Polycast, and my interpretation is that they didn't feel Bushido is necessarily weak, its that it doesn't alter your playstyle, and its effects aren't directly visible. So for a moment ignore how "good" or "bad" all UAs are:
For example with Bushido, you don't change your play by attacking. It just means they attack harder. Strategically this alters you to play more aggressive, but the way you play doesnt change. Contrarily with venice, you must play the game different than everyone else (regardless of strategy). You dont build settlers like everyone else, you buy out citystates. Changes the type of play in addition to the strategy. Also with Polynesia, you get to actually do something different with your UA. You get to travel where no one else can that early. Again, ignoring how good or bad it is, it changes the way you play them. Also with them you get a whole new improvement that, regardless of how good or bad it is, makes something unique and forces you to play different.
I think thats what they're going for with a change for Japan. While fighting at full strength even at a sliver of health is a benefit, it just adds numbers to an attacker, something that isn't "seen" by the player, and doesnt change how the game is played by Japan.