I think the biggest flaw is that it is per trade partner, not per trade route. In my experience at least, i always end up with most of my trade routes going to the same civ on the same continent (who is usually the strongest with the biggest cities). Forcing me to trade with subpar partners to get the UA bonus makes me lose out on a LOT of gold, and 1 culture is just a drop in the bucket.
How would you change the UA other than just increasing the bonuses?
How would you change the UA other than just increasing the bonuses?