On the topic of trade caravans...
As far as I can tell, if Trade Caravans haven't been given a buff in 2.8, in the early game, trade caravans barely gave you 1

per

, and later on they give you around 1.5

per

.
In other words, they are BARELY better than building wealth! I am reasonably sure that if you built wealth instead of trade caravans, you would find a similar result (not as pronounced, of course, but still significant)
Trade caravans are slightly worse than wealth, however. Firstly, they take time to create and move. Next, as military units, you have to pay upkeep on them (varies between 1 GPT, and 3 GPT, depending on civics). Finally, Trade caravans can be captured and killed.
So, no. They aren't horrendously over powered. Sure, they ARE strong, but they do have major weaknesses, and aren't all that different from building wealth (Just a slightly better hammer:gold ratio, slower rate of gathering, and slightly less reliable)
Ideas for balancing:
+1 GPT upkeep cost (Freights might cost +2, or +3 GPT, and be significantly faster)
OR
Reduce the gold gathering rate by 10~15%
OR
Impose a 5 Unit National Limit on building them at a time (Wasn't this automatically in 2.71 anyway??)
Thus they remain slightly better than building wealth, while still having significant draw-backs.