From my understanding, the Formation Infrastructure family was generally seen as subpar. They used to give you a rather small decrease in Unit Industry cost, which means that you'll need to create a large number of Units before the cost reduction begins to start paying itself off. I believe that's why these infrastructures appear to have been buffed, such that the Garrison now provides 10% Unit Industry cost reduction, and the Tourney Fields provides a 15% cost reduction (in exchange, the Money cost reduction was removed). 

I believe that similar to the Phoenician and Carthaginian LTs, these cost reductions will stack up additively, rather than multiplicatively (so two -25% cost reductions will give you a -50% cost reduction, instead of a -43.75% cost reduction). However, a problem with an ability that reduces the cost of something is that it can be stacked up to large amounts, potentially up to a dreaded 100% cost reduction. And at the moment, it is possible to get a 100% cost reduction. 

It turns out, these cost reductions don't stack additively. Whoops! You can probably go ahead and ignore the rest of the post if you want to! However, I still maintain my stance that stacking up Unit Industry cost can be a valid and rather powerful strategy, although it's definitely not as powerful as I had initially believed. 

There are currently three different Cultures that provide Unit Industry cost reductions. Aside from the fairly common complaint that these Legacy Traits feel too similar, they can also be stacked up to devastating results. The Mycenaeans, Aztecs, and Soviets all provide a -25% Unit Industry cost reduction. Altogether, this is a -75% ≈ -57% cost reduction, or four Units for the price of one. However, it's possible to get to -100% cost reduction even before reaching the Contemporary Era. 

The Conscripts option of the "Army Composition" Civic can allow you to get -20% Unit Industry cost. The Barracks Infrastructure gives you a -10% cost reduction, and the Tourney Field will give you a further -15% cost reduction. As early as the Medieval Era, you can get a -95% ≈ -65% Unit Industry cost. Jaguar Warriors are naturally a bit of a swarm Unit, given that it has half the price of a normal Great Swordsman in return for slightly lower Combat Strength. With a -95% Unit Industry cost, you can get 40 Jaguar Warriors for the same price that a normal player is going to get a single Great Swordsman. Needless to say, this is potentially a bit powerful. 

In the Industrial Era, you can finally get that final bit of cost reduction, via the Military School. In the Industrial Era, you can gain access to the Military School, increasing your cost reduction by a further -20%, or turning our running total into a -73% Unit Industry cost reduction. You can gain access to the Military School in the Early Modern Era if you take a Scientist Culture - and after that, you can rather easily spam out a large number of Units and conquer the world. 

If you reach the Contemporary Era, the Soviets will add on an additional -25%, allowing you to get to a total of ≈ -80% Unit Industry cost reduction. Combined with the Red Army Tank, you'll effectively be getting 10 Units for the price of one. With an endless horde of Units, it should be rather simple to conquer your way through the world, if you haven't already done so. 

A game involving this will obviously need to include the Mycenaeans and the Aztecs. You also want a Scientist Culture in the Early Modern Era, of which there is only one - Joseon. In the Classical Era, the best Culture will likely be the Celts, since you'll require a large amount of Food and Population to be able to sustain the endless Unit spam. Past the Early Modern Era (of you haven't already won), a few good choices might be the French, who can allow you to get Contemporary Era Units, the Mexicans, for Mother's Milk to sustain your constant Unit production, or the Germans, Russians, or Zulu, who will all make your Units more effective. 

If all these mechanics do work together in this way (and I must say that I hope that they do not), then this may not be very good for the game. If nobody will mind me temporarily turning this thread into Game Design, a possible suggestion is to slightly change the Unit cost reduction mechanic. A simple fix would be to make the reduction multiplicative rather than additive, so you can stack it up and up, but you won't ever be able to get to -100% cost reduction. However, this can potentially be a bit less intuitive, especially if the Money buyout cost reduction is kept to be additive. 

Another suggestion is that instead of making it so Units will cost x% less, it can instead be that you get x% more Industry towards creating Units. For example, rather than a -25% Unit cost reduction, it can instead be +33% more Industry towards building Units (it can probably be increased to +40% for a nice round number). In this situation, getting more and more +% Industry towards building Units will have reducing gains, rather than increasing gains. This will avoid the dreaded -100% Unit cost reduction, while still keeping the idea of making it easier to build Units. If wanted, several sources of Unit cost reduction can keep it - for example, the Mycenaeans can get additional Industry to building Units, while the Aztecs can keep the cost reduction. This can make the LTs somewhat more distinct, although it would be good to get additional bonuses to make the Mycenaeans, Aztecs, and Soviets more different from each other. 

Given that the cost reduction already stacks multiplicately, this paragraph is moot. My only suggestion at this point would be to reduce the sources of Unit Industry cost reduction, so stacking it up to a larger amount would be more difficult.