ya, if you use it right... it gives... i wanna say 75 fleet supply worth of stuff.

This ability gives you a superb number of units for a very reasonable price.  As has been mentioned, this presumes you have open fleet command for it to fill.  There are three issues it has beyond this:

1) It's very expensive to research, and that's not even considering the cost of the phase stabilizers themselves.  You need at least 4 or 5, if not 8 or 9, for this ability to be worthwhile.

2) You don't control which types of units you get, so it's somewhat of a gamble that you'll get good unit types (Assailant, Sentinel, Carrier) over bad types (Enforcer, Skirmisher). 

3) The Kostura is probably the most effective "match-ender" weapon in the entire game, and it's easier to unlock than this ability, so it's kind of redundant in that sense.

RA never gave capitals.....ever

ok the more i use this the more i feel that its a rip off, first you need to get both levels of the phase tunneling speed increase, then the ability itself only seems to be viable at level 2, and finally it costs 2500 dollars and 500 crystals (!) to use once (i used to think it spawned multiple ships over time). is it actually less cost-efficient compared to building the ships yourself, especially if you have both slave labor and optimized construction?

No, it's much more cost-effective than building units.  This is the whole advantage of the ability. 

Not quite so bad since their damage buff, but still a questionable unit. 

Biggest problem is that they don't use phase missiles, which is a big deal late game, especially against Advent.

As I've said several times already, illuminators were essentially god last version, and when the bug causing them to deal unreasonably high damage was fixed it was revealed to be a rather mediocre unit in reality.  I suspect next patch will probably buff them.

so, as advent its better to live on guardians and expensive carriers until i make it tier 5 military and destras? will this also work in multiplayer? what if they get flaks

Returning Armada is a "vestigial" tech.  It used to be a game-ending technology in the original Sins v1.05, but it was severely nerfed when Entrenchment was released.  Before the nerf, the phase stabilizers could be set to operate automatically, giving you FREE ships ever couple minutes.  So, if you were Vasari, you would makes ~15 phase stabilizers (can put 2 on an asteroid, 3 on a planet) and let the free ships just pour on in.  A Vasari player could thus quickly ramp up to having 2300 fleet supply worth of ships.

In multiplayer games, people might sometimes type out: "I have Returning Armada, this game is now over."

Vasari's problem was actually being able to get to the point where you could afford to research it and build all of those phase stabilizers.  If a team in a 3v3, 4v4, or 5v5 started out having a Vasari player in a safe eco spot, the guy in the eco spot would often rush to get it up.

Few people ever use it or pay attention to it since the ability was nerfed.

it prolly could be slightly de-nerfed, but not much. if it was 2000 credits, and 400 crystal, it would prolly be ok.

What we cant have is that scuttling the ships gives you equal or more monies than it cost to generate them... which happens at 50% off... and getting 50% off ships is not bad.

ya, i tested that, it brings the scuttle percentage to exactly 50%, which is why i said 50% in my pervious post.

NOW, mind you, if you research the 'cost and resource decrease' techs, your return is diminished.  Its too bad that (i dont think) abilities can be told to honor research.

RA never provides Overseers and Subverters, so its not like you can just build a bunch of Phase Stabilizers and use RA and expect a well rounded fleet (what you get instead is a highly brute-force fleet).

Well thats fine if you wanna buff it, but then I want my Insurgency not to be xp fodder. Or the Novalith upgraded to actually be useful.

If your gonna justify buffing RA just b/c it cost alot of labs and research then, why not start with already lackluster tech?

RA, while not as great as it once was is still great. You get ships at a reduced cost, you get them all at once. Having a few extra meaty ships join the battle at once is not a negative thing, even more so if your fighting an attrition battle. 

While you cant choose what ships you get you do know what types so you can use your frigate factories to build so you have a balanced fleet.