I looked for this info and couldn't find it, so I tried it in my current game. I'm Venice and thanks to city-states, I'm already at my supply limit. I took Volunteer Army and all of a sudden, I'm -60% hammers. The increase in supply was the only reason I was interested in the tenet. If the Zulus finish taking the rest of the world, I'll abandon science and march into Ulundi, and the six extra units would make that easier. I have all the money I could want for anything other than busing spaceship parts, and I have most of that, too. I could financially support whatever units I want. The +20% CS is nice, but unless and until the time for war arrives, it's not worth taking a tenet and then having to give away six other units just to keep building wonders.