Geese are good as food resource indeed. BUT:
- They require a lot of micro. At some point this might become too much, and then you will run out of food. If you run out of food, then you will also be unable to spawn your food source, that's an important inconvenience.
- Romanians and Greeks have not access to another herdable.
- Pigs/Sheep fatten slowly at hunting lodges (techs don't work)
- Romania can become very food-heavy early, when you spam out villagers. Geese cannot follow this speed. Or you sacrifice on villager creation, or you plant crops, or you do some risky hunts.
In the current version, most Balkans civs are too weak to win late game (30-40 or later). Except for the Greeks.
Serbia's Chetnik is a bit UP, and all other infantry is useless. Both late UU have low HP, which increases the benefit of the UA, but also puts a burden on your (food) economy.
Bulgaria's army has three major weaknesses that are difficult to overcome: skirm weakness, slow train time and population issues. This makes them bad late game. Crops are usually not important while playing as them.
Romania has nothing special late game, and has no typical skirmisher. I don't even know how I would counter dragoons with them. But dragoons are common late game. So you will probably have to spam?
This leaves only the Greeks. They are the boomer of the three. But on land maps, they will also need crops, since you have a very good but very food-heavy UU, the Chorophyllax.
I no longer know why I wrote that minor overview :-/
I just want to finish with this: Balkan civs typically must win within the first 30 minutes. You don't have much time to heal units, you may not boom for long. If you run out of hunts, you cannot risk waiting on food from geese, you will need to place some crops, preferably you placed them early enough.