I have been runing 2 games with Sophon and Lumeris with an easy score victory with each (even easier with Lumeris tho) but Vodyani seems impossible to play to me. Even while being over agressive I couldn't develop myself fast enough. Essence is slow AF to gather, not to mention that you absolutely need a target system. Industry is weak, so dust generation is. I don't really understand where the power of this faction lies.

Don't know. I've only played Lumeris and Cravers so far. I love how fun they are to play in ES2, whipping slaves and working them to death.

Ye I found the same thing. harvesting essence was easy once I had a fleet of 4 dedicated ships over an enemy planet (I was getting 138 per turn) but up to that point I only had 3 pop and one of the other factions was nearing score victory on turn 50 (this may be because I ended up confusing myself and not building any food at all). 

Also the fact enemies can jump to a system and attack your Ark and you cant do anything to stop it was really annoying. I was under constant attack from another Vodyani group aswell as from a minor faction that declared war on me 20 turns in. Because my fleet cap was only 4 and an Ark takes up 8 I couldn't add any extra ships to defend it and enemies could ignore all other fleets in the system and attack it straight away.

One cool thing though was I found the harmony in a quest line :D cant wait till they get their own models because atm they borrow them from other minor factions.

I think they will fare much better in the full game, as once you get a few defended extractor ships out to colonies by turn 80-90 you can get enough essence to become a powerhouse. In early access, it isn't soon enough to make much of a difference due to being behind on pop, colonies and military.

The Vodyani require a very different playstyle (as races in Endless Space tend to), there's this thread (not by me) which explains how the Vodyani are actually pretty OP: https://www.games2gether.com/endless-space-2/forum/66-game-design/thread/20795-balancing-the-vodyani

I haven't played them myself, but every game I've played so far has ended up with the Vodyani being ridiculously powerful and extremely aggressive, so it does seem they're pretty strong right now.

They seem like they could be very powerful. but every game i've played so far i've had a fleet sneak into my systems and ground assault it with ease smoking the arcs in 1 turn each, they have very bad ground defense it seems and the arc doesn't defend itself if attached.

Could be that i'm nut understanding ground combat fully, or that's just their intended weakness. But i get steamrolled. 

I just ended a Vodyani game, with a defeat, but I was second on score, and I found them relatively strong. It took a little getting used to (first test game was a bust), but once I started aggressively going for essence almost from the word go, I could get a new ark out by turn 15-22 on normal speed (depending on closeness of an enemy civ), and with that ark everything was a lot better (if you need to crush someone, you have a truly powerful cap ship to do it, along with whatever fleet you have, if not, you have a way to bulk up for later fleet building, etc...). For reference, it was a 4-player game, one of each race, and I was at war against Cravers starting around turn 32, and the other two became constant combatants (when they weren't screwing with me with truces) around turn 75 (Lumeris first, then Sophons whenever Lumeris tied me up with a truce). My military might was truly massive, and I could pump out full 9+ ship fleets in 3-5 turns once I was in era 2, despite having fewer colonies than any opponent, except my early dominated Cravers.

There are a few issues I had with them, some of which have been mentioned. 

The most glaring is the ground assaults. It seems nonsensical that they can be ground assaulted without a fleet engagement first, since even if I were to assume they are completely stationary in orbit like a geosynchronous space station, they should still have the ability to fire at oncoming attackers. Something needs to reflect this, whether it's a power/feature that dramatically cuts down on attacking manpower automatically, an automatic fleet battle against the ark before the ground invasion can go off, or something. I understand that being ground assaulted more easily makes them more like planets (despite lower manpower base), but it also makes no sense and was really glaring especially when I found out arks die after ground assault rather than doing a forced retreat (which I would argue is another, but inferior, alternative way to somewhat mitigate this thing). Hopefully this is an EA thing.

Second, with the turn limit essence generation definitely has different peaks than it will in a more full game, at least it feels that way, with 4-6 arks seeming to be the best you can hope for without making combat fleets more expensive (and weaker) by adding harvesting tools, or having to invest in the long-to-build, poor at defending themselves, Leechers. It's a bit tough from this skewed feel to try and determine how well they'll escalate into lategame, but right now I'm feeling they'll get reasonably better the longer it goes on, able to win militarily, or combining military and other conditions (which is technically what I tried to do keeping opponent scores down by wiping planets off the map, since I didn't have time to wipe them out). As a side note, I'm curious if essence will have any use besides arks and population. 

Third, upgrading arks feels strange at the moment, and I'm curious if it is planned to be tweaked. On the one hand, it's cool to balance moving older arks up for their pre-existing infrastructure, versus using new arks that are made to schematics that are greatly superior, but it feels forced heavily in the current build to do the latter for safety without a fleet handy, since producing dust has been tough for me until Era 2 hits, with its research. In fact I couldn't upgrade arks ever in one game because I had to have too many fleets to insure I was getting essence and wiping out the quick-breeding Lumeris colonies near me (and their fleets) just to survive. The dust price is pretty crazy high for even simple upgrades of the basic weapons, especially since in many cases those upgraded arks are going to be a) unused as weapons since they're already upgraded and are more valuable as colonies b) will require defensive fleets anyway due to the way the ground assaults of arks are currently managed.

Fourth, even pushing essence production as high as I can early-mid game (without crippling warship production for protection, and development production for infrastructure), Lumeris and Sophons can both out-colonize me by leaps and bounds (anywhere from 3:1 to 5:1 typically), and I was wondering how correct that was, since it was an obvious impact on score (especially since score seems heavily biased towards population, and colonies).