If you have a "slightly stronger" unit, you're obviously supposed to strike while the iron's hot and start rushing. But once that unit is obsolete, your advantage is gone and nothing promotes upwards. Nothing wrong with that. It's a design choice.France have an awesome UI, Greeks companions can wipe the floor of most classical UUs + their UA is strong, Legions & Musketeers are good as they stay for far more longer than Samurais do (Muskets come very quickly after LS) & so on. On the other hand Japan has a meh UA & meh UUs which don't compensate for its weak UA. I agree that Samurai are not the worst UUs but they aren't certainly good enough to make up for a weak UA & 0.
Also what do you do with your GGs after you get 2-3? Most of the time they sit idle or rarely used for building citadels. Maybe I should start building more citadels but anyway I don't find GG promotion extra exciting.
I do feel that Japan could use a little spicing up, since the UA is not as awesome as it once was, and the UU's are just serviceable. They're "terrible" in that they're not awesome, and they no longer have an awesome UA to back it up.