I am not sure what you are refering to. But PDOX games have a well-established history on how to make games more challenging if you grow too fast. Be it increased corruption as seen in EU:Rome, be it friction between nationalities as in Vicky2, be it demesne juggling like in CK2, etc.
This Unity -> Tradition is quite comparable at how fast you gain leaders or diplomats in Vicky2. If we took the Unity -> Tradition to Vicky, Haiti would have the greatest military leaders and the smoothest diplomats in the world, because as a small peaceful country it would have high Unity generation and thus could pick the suitable Traditions with Unity. Meanwhile a Prussia which turned to Germany through diplomacy and wars would have lousy diplomats and lousy generals, because obviously a rapidly expanding country will have low Unity and thus can't pick up the required Traditions for good diplomats or leaders.
A game with such a mechanic would just stink, wouldn't it?