In the last 2 weeks I have been playing Endless Space 2 like a fanatic :p, been a long time I had not played this much a 4x game (been around during the beta, but back then I lost interest due to how bland the game looked at the time). That being said, I noticed not all factions perform equally under the AI's guidance, some perform so bad (cravers) that I decided to not include them in my solo games. I want to share my findings and see how well or poorly the AI did for other players out there and maybe try to figure out what isn't working properly. All my impressions are playing against the AI at medium difficulty. Did try a hard game once but the ranking seem to remain rather similar.

Factions

The Riftborn

For some reason, the last few games I played, the Riftborn always end up being the most powerful faction among the AIs. Most planets, better fleets... And while they are supposed to be pacifist type, they end up way too often in wars with most of their rivals.

United Empire

Only added it in my last game since i'm actually playing that faction, but they seem to do alright score wise. Yet they have a pitiful spacefleet and often use minor factions ships rather than their own ships. Not sure why they struggle with building up a worthwhile fleet, they have more systems than me and I completly outproduce them.

The Horatios

Every game I played they where there, and scorewise they were among the strongest. They have lots of colonies and have a good score. But their fleets are weak, they fold rather quickly when attacked.

The Unfallen

So far, they always end up one of the weaker factions. Few contact with them so I don't know what they are doing.

Sophons

Strangely enough, the brainiacs seem to not keep their edge technology wise, they often end up being one of the weakest factions. 

Lumeris

After the Riftborn, the lumeris seem to be the strongest each game. For a peace loving, trade centric faction, they seem to grow rather huge, field plenty of fleets and not be a push over when bullets start flying. Strangely enough, i've seen them declare war on me early game while I thought they were not allowed to.

Cravers

Had them every games at first until I realized they were both pushovers and also a major pain to conquer since I don't want their pop to wreak my worlds. For a war/expansion centric faction, they hardly deliver and sue for sease fire way too often. They fleets are easy to deal with and if we stall them long enough, their depleting worlds mean they will drop down good last in the ranking and be a really weak faction that can't do much.

The Vodyani (Forgot that one)

As far as I can tell, even they manage to expand they are hardly a force to reckon with. Their moderships are juicy targets they often leave unprotected. They spam their little ships but hardly field any worthwhile fleets. They are also the most annoying diplomacy wise, and they declare war when they have no fleet to speak of... 

So, what about you?

This certainly changes a lot when on Endless difficulty, as the Cravers are by far the strongest faction in the hands of the AI at that level. They will with high reliability steamroll any other AI in their way and likely you unless you engage them early enough to contain.

Your impressions are otherwise fairly accurate. Lumeris and Horatio are most consistently the best in my games, followed by UE and Riftborn. Sophons is a toss-up; sometimes they do exceedingly well, sometimes they do not. Vodyani never do well, and Unfallen only do well if left alone.

FWIW, fleet power doesn't really vary much. All the AI are bad at combat, with some rare exceptions. This is exacerbated a lot by the fact that the AI is coded to 'play the game as it's meant to be played', while players can easily break the flimsy combat balance.

I am honestly having trouble understanding why it is necessary to play Endless difficulty to get a challenge out of the AI. 

In my current game, the AI I have come into contact with has expanded well, has a ton of resources and has absorbed several minor factions (both Horatio and Sophons). What exactly is preventing the AI from fielding decent fleets at lower difficulty levels when they have the means? 

I'm not necessarily talking about ship design here. I mean researching CP and hull techs and using weapons that they have resources for, then sending sizable fleets at me. The patch notes say that the AI can now build invasion fleets. Why am I the one with the unenviable task of clearing out pirate bases then?

I was struggling with Vodyani. I hadn't played them in a long time and lost a couple of Arks to pirates. At the same time I was being a thorn in Horatio's side and pressuring him to give me stuff. He never declared war on me. I was weaker and was lagging on building fleets because I was researching a lot of colonization techs.

I keep seeing way too many Explorer ships from the AI instead of war ships.

Endless Difficulty (Endless Speed with Max faction count and pirates) 

1. Cravers and Riftborn 

Both generally become the strongest factions by mid game (roughly turns 200-300) but neither seem to make Alliances and end up getting swallowed by other factions and thus I've never seen them by turn 400. 

2-4. Lumeris 

They expand extremely quickly and usually make an alliance with the Sophons and the Unfallen (sometimes I see them ally with the UE). However, their fleets generally aren't that good, and typically choose diplomatic actions and trading over war (since they're capitalist pacifists). I hate being in trade agreements and alliances with them since they always find a reason to eventually declare war, and/or force me to war when they absorb one of the minor factions in my empire. 

2-4. Sophons

By far one of the strongest late game factions (if they make it that far), this is in part b/c they gain access to a majority of the technologies first and since scientific laws are some of the best in the game. I've seen them survive if they make an alliance early on in the game, but if the Sophons become isolated they generally fall off relatively quickly.  

2-4. Horatio

Also pretty warlike once they reach late game, and they always reach late game. But expands extremely quickly and generally wins wars against other AI's, though I can't see why cuz their fleets are pretty bad. 

2-4. UE

So, damn, annoying. Aside from the Cravers, the UE declares war the most. They almost never choose the Mezari path and a number of times I've seen them go for the Sheredyn path, which I would assume, makes them more warlike (in the AI's decisions). However, their fleets aren't too much to worry about and fold pretty quickly (except for that single one led by Hadri Lenko, b/c of the bonuses he provides). 

5. Unfallen

Though they can dish out some pretty strong fleets, they're hampered by a slow expansion rate and I've only been in two games where they actually begin to pose a threat (it is important to note that they ended up dying once they expanded too quickly, I killed them both times since they spawned in the same constellation and were kinda in the way). 

6. Vodyani

Also hampered by the slow expansion rate, and even though they somehow always manage to survive until the end (even after declaring war on everyone ???) they stay relatively small and have never posed much of a threat. 

By far the weakest AI faction.