I think people have a different definition of easy than I do. I think the AI overextended itself which made him lucky so he was able to counterattack, but he not only had two cities taken (losing his entire airforce in the process) but he was one turn away from his first city founded getting burned to the ground. Now I agree that the AI should have stopped after taking two cities to make sure reinforcements came and the city successfully burned down, but I still think he hung on by the skin of his teeth from all appearances. And it's unclear how much he was set back by his losses (certainly, Brasilia had a far fringe tech relatively early because he stole it).
That being said, sending a trade route from each city to the capital is apparently quite strong (on the other hand, almost every minute he complained about lack of energy, which might have been remedied through that extra trade route). Plus, one thing I noticed is that things are balanced by providing a ton of useful things. Sure trade routes are really good, but there are also a lot of buildings you want to squeeze in instead, and lets crank out some explorers and get some expeditions, and oops, maybe I should have been building military units because they just declared war on me.
Regarding negative health, I agree with the thrust of what people are saying but I'm not sure I agree with the specifics. I don't think the game is designed where the ideal state is always being healthy. Healthiness is an ideal to shoot for but mild unhealthiness is not a problem. Health simply doesn't appear easy enough to justify positive health. The problem is there needs to be even greater penalties for higher negative unhealthiness. If you can compensate to live with -20 you can currently go as high as you want. Instead, add new penalties for -25, -30, etc. (for that matter, -15 as well). Although it might be more balanced in multiplayer. In a multiplayer game, if I see an unhealthy player going on a war of expansion, I'd put spies in his major cities and try and flip them.