Overview
There are only two ways in which you can fail completely:
Happiness failure: Your empire falls below 60% happiness.
Money failure: You have run out of cash and are bankrupt.
However, you can also "fail" in these other ways, so I will address them too:
Bad starting planets/difficulty expanding
Not enough research to get techs
Lack of trade material and connections inhibiting success/prosperity
Happiness Failure
Your empire starts at 100% happiness and fails when it falls below 60% happiness.
Now, what makes it go down so low? This will only happen if you have planets that are in the red "struggling" state (lacking the resources for their level 1 growth). When a planet is struggling it will reduce happiness by 1% each turn down to a maximum of 8%. Unemployment adds an additional penalty per idle worker. When you fix the planet by connecting resources and reducing waste, the penalty goes away.
These rules are made to make happiness something you can take a "temporary debt" on, just like money. If you have a planet you know you'll fix in the late game, you can let it slide down to 8%. On the other hand, if you want to expand aggressively you may want to let a few planets struggle and then fix them a turn or two later, "resetting" your happiness. So as long as you keep expanding enough to fix the unhappy planets you can stay in the negative for a long period.
Of course, you can also aim to boost happiness to counter the negatives and to improve your endgame score. Improving the economy will give you a little bump, but happiness-boosting techs and structures can do much more.
Money Failure
The other way in which you can game over is to go bankrupt. This is the main way in which higher difficulties are harder - you run into money trouble more easily.
Like with happiness, fixing your struggling planets is one of the first ways in which you can improve your money situation. You get bonuses for keeping the economy balanced, so linking two planets to consume some idle resources can get you a little bit of profit.
In the early expansion stage, it's as simple as keeping the planets happy. But it gets much harder once you have a big empire and upkeep starts costing large amounts. This is an important strategy decision: Are you going for "wide" (lots of minor planets) or "tall" (a few planets that are all economic juggernauts)? A big empire will need techs that apply globally to overcome money issues, while a smaller one can focus on maximizing quality by using more constructions.
Making The Best Of Your Starting Position And Expanding Safely
It's tempting to think that the game is all RNG, especially when your starting position fails to give you a nice balanced route.
However, when this happens, you can take calculated risks. First off, let's review the two mechanics you can use to guess-and-check:
Pinning (right click on the planet build order instead of left clicking it) lets you get a sense of a complicated route configuration.
Undo (U key) lets you take back moves that didn't involve revealing information. This means launching probes, viewing the council missions, and connecting the abandoned labs. It does not stop you from colonizing to see how populated the adjacent part of the map is, which could be a useful way to min-max your early game.
When selecting the planets to colonize, there are four rules of thumb I use:
Colonize right at the edge so that you can see a larger area
Factor in some "unhappy planet" debt. Remember, if you connect the planet it'll remove the penalties.
Aim for a "resource profit". If you can get one of resource A or two of resource B, you probably want to take the surplus of B unless you have a good reason to expect shortages.
Aim for a "resource cycle". In each region of the map, you ultimately want to have some of each resource, versus having a lot of resources stuck on one side of the map unable to be delivered to the other. You might not be able to connect every resource immediately, but in the high-scoring run, you would have a nice flow.
Making this calculation of what your routes will need is really 90% of the gameplay, and it's all dependent on your planned empire size and techs - you want to have a way to reduce your risks plus a way to accelerate when things are going well.
For example, if you plan to take Enlightenment, you'll be able to eliminate Goods from your routes, which means you can use your Forgeworlds for other things and get a larger population. But if you have a larger pop earlier on, you will need to also take something that can consume them so that they aren't idling. Finding a way to both get a surplus and then use it is the key to speeding up your progress.
Speeding Up Research
It can be frustrating to build labs and then not get much benefit out of them. It's an art within an art to get this part of the game down.
First of all, the game really wants you to dedicate one lab to each type of resource through penalties when you have multiple of the same kind. And resources count for twice as much as pop contributions. This means that your prize-winning labs are likely to be "resource bombs":
Rather than try to plan a balanced route, take a group of planets that can produce the same thing and just colonize all of them and let them be unhappy for a while. (You'll fix it later when you get the tech.) The downside to this is that now you also have an unbalanced clump of resources that are likely to never find delivery...unless you have SO MANY planets doing this that you can tank the same-type penalty and run two labs!
Once you add in perks, tech, and council mission bonuses into your research plans, things get a little more spicy. For example, if you have a bunch of habitats, you can push more pop into one lab than if that pop were stuck on a single planet. There's a tech that gives each lab a bonus, which could be used to counter the same-type penalty and spam small labs. If you play with the Vattori you can rack up a lot of research just through mission bonuses.
Lack Of Trade Material And Connections
Improving the planets is really a two-step thing. Making them "not unhappy" is step one, but getting them to "successful" status can be much harder or impossible if you didn't plan out a "resource cycle" in the region. (as described previously) This is especially true in an area where you set up a lab with a resource bomb; you got the tech, but now you may spend the rest of the game doing too much surgery to try to fix up those planets.
It comes down, again, to knowing your options. It gets VERY difficult to understand what you could optimize in the late game if you're swimming in options for plunking down more routes or adding upgrades. The problem is that if you already have a big empire you can't actually add a lot because the upkeep will get you, so really, you want to select a particular category of "problem planets" that you'll address right at the end. I am often surprised by apparently low-value techs that do exactly what I need, and as a result I will often try a tech and then undo just to see if I get more from another option. Techs and perks that give flexibility when colonizing in the early game can be a bit of a double-edged sword because they may actually hinder reaching successful status, especially if they lead you towards a homogenous empire that produces a lot of the same stuff.
Anyway, the above tips should be enough to get you to a win screen.
Source: https://steamcommunity.com/sharedfiles/filedetails/?id=2510151131