Playing on The Great gives you a massive -12 happiness per turn in all of your cities from the start of the game. This is probably where the biggest amount of extra difficulty comes from, arguably even more than the massive headstart the AIs get with 4 settled cities and 4 extra techs.
The happiness penalty slowly strangles your empire with escalating debuffs that interlock and reinforce with each other. Higher discontent means families lose opinion. Lower family opinion leads to rebels spawning that will randomly attack your cities and lead to even more unhappiness. It also causes a reduction in combat stats, making it harder to defend against random spawns. Beyond this, unrest also increases maintenance, both from itself, and from lower family happiness, meaning you'll need to devote more of your empire to making gold to stay afloat. This mostly trades off with making science, the resource you need to scale, and discontent also comes back around again to directly reduce science output on top of that. The rebels are what can kill you outright, but it's a nasty package in a ton of ways beyond them.
The weird thing is how you have almost no ways to cost-effectively counteract the happiness drain in the early game, nor even in the mid game unless you rush for it. Spamming festivals is a terrible waste of build queue time. Walls can give you +1 happiness per level, but do little else for cities that aren't on the frontline. Luxuries can help a little bit here and there, but are fairly rare even with the plentiful resources world option, and at just +2 happiness per lux an empire of 6 cities would need 36 luxuries just to tread water -- not even remotely feasible. Governors can give a tiny trickle of happiness here and there but we're talking like +1 or +2 max, and they're expensive in terms of civics to slot it.
If you do a bunch of these options simultaneously they can slow things down a bit, but they're so inefficient that they're rarely worth doing for their own sake, and they won't even come close to actually solving the problem. Because of this, it's actually usually the better option to ignore happiness entirely, treat it like a ticking timebomb, and rush for the actual solution.
The correct solution to happiness is religion, specifically rushing the Monasticism and Doctrine techs. These are tier 4 and 5 techs, so you won't be getting them early if you don't go out of your way to do so. With them you basically fully unlock religion all in the span of 2 techs, and get a much better happiness economy while also getting loads of other goodies. Tolerance gives you +2 happiness per (non state) religion, Enlightenment lets you get +3 happiness from monks, garrisoning a disciple on a Temple gives you +1 happiness, and Priests give another +1 to +3 happiness. Then the kicker is that you can swap to another religion, reform it to have the right traits, and get all the bonuses a second time! Along the way you can slot in the Gnosticism tenet for +1 science per urban specialist, which is a significant boost to science output, so you're continuing to scale your eco while you do all this.
This is a very strange way to balance the game. Old World has a bunch of design choices that are strange compared to other 4x games that nevertheless work very well, like the Orders system. But what's the deal with happiness being a timebomb at high difficulties? Why is basically every other option rather than abusing religion a false choice? Why would you ever go for the dedicated happiness buildings, baths, when they're basically unable to do their job? The only other slice of techs that come close to the religious duo is the line of Citizenship - Scholarship - Jurisprudence where you unlock like 70% of your late game scaling in the span of 3 techs. Most other techs seem like filler junk that just gets in the way.
