There've been some interesting reviews of these extra units so here's my reasoning on them:
All Units: Keep in mind that you are limited to having three of each unit active at one time. This tends to limit any of the excesses that are even possible. You can't form up a huge set of Supply Trains to instant build a wonder. Nor can you have waves of Caravans supplying you with gold. And you can't have large numbers of Entertainers building up a city's culture unduly (in fact, given that Entertainers provide 15 culture and Great Artists provide 4000 culture, it would take 266 Entertainers to provide that same amount and would cost 10,640 production to build them... or about 7 times as much as the Eiffel Tower just to get the effect of ONE Great Artist).
Caravan: No, this doesn't replace (nor should it) the trade routes functionality. It is intended to provide an
option regarding commerce and nothing more. The return on using the Caravan vs. just setting a city to build wealth is designed to be about 20% better. Nothing so huge as to provide an overly large return given that, at least in my experience, it's uncommon to just leave a city on the build wealth setting. But it can be handy when you're needing a small temporary boost in available gold.
Supply Trains/Freight: These seem to be the one type of units that everyone actually likes.
Entertainers: As noted above, these units are horribly inefficient for developing large amounts of culture. They are good for helping a very new city (or conquered city) get on its feet. One Entertainer will get a city to a radius of 2 squares. It would take 7 (and remember you can only have 3 active at a time) to get a city to a radius of 3 squares. The intent is to represent the effect of a government deliberately encouraging the growth of their culture through small efforts rather than taking the small frontier town and building an opera house in it. Think minstrels, bards, circuses, and travelling road shows. If they are still considered to be too powerful as they are, I may drop their benefit down to just 10 culture.
Other Units: Since I put up this mod, I've been getting requests to make a food transfer unit. And believe me, I'd love to do it. The only way I've been able to come up with something like this that would actually work is a bit roundabout but it's interesting... A pair of buildings set to work like National Wonders (i.e. you can only build one of each). The first would be Food Export and would lower the available food in town by say 4. The second would be Food Import and would require that the Food Export building had been built somewhere in your empire before it would provide its benefit of say 3 food (some would be lost or spoiled in transport). This would enable a single city to help support the growth of another city. It's
possible that this could be enhanced to allow it to work for multiple city pairs (using roughly the same kind of python code that checks to see if you have enough temples before building a cathedral) but I don't know for sure. I'd also like to see if there's a method that the buildings could be torn down so you could rebuild them for a different city pairing. Any thoughts on this?
As for merchant/freight ships and planes, you can always board the units onto a ship or use an airport.