If I had to do some crystal ball work on what the balance would be like, based on my experience in WAR and GW2 on the underdog and overdog sides, and the general rule of thumb that you'll find an above average number of veterans amongst the members buying out of free to play, then I'd expect that the SM faction will have the highest number of veterans because its likely to have the highest number of paid players. So for the first few campaigns they'll be vastly more effective in the field due to the high volume of players and correspondingly high volume of good players.

Then something odd will happen.

The Orks veterans will start to marshall the F2P players into a more effective green tide and after stomping Chaos and the Eldar as practice runs, will hurl themselves directly at the Space Marines in a bid to end their domination of the server. The Marines will be thrown back, as much from shock and awe as anything else. Whether its counted as a true Ork victory or not, it will feel like one to the Orks, and the next campaign will see a bloody fight between the two as the Space Marines try to recover their wounded pride and the Orks ride high on their morale boost.

Then the Eldar will suddenly become kingmakers. Having been stamped on by all sides from being outnumbered, their remaining players will have by this point have got a higher number of veterans per player ratio (you tend to get a lot of practice at difficult fights when you're forever outnumbered and outgunned, and that generally turns you into a better player than being a hanger-on in a dominant faction). The smaller Eldar force will turn the tide against whichever side it attacks - either the Orks, for having recently stomped them, or the Space Marines for having stomped them so many times before.

In the subsequent campaign, both the Space Marines and Orks will appeal to the Eldar for a formal alliance against the other in a bid to retain / achieve server dominance.

At which point, having finally been given a chance to breathe and organise (Chaos can be a bit slow-burn at this), the first EC Black Crusade will deliver an absolute sucker punch to the Space Marines. Because, frankly, who else are you going to turn it on? The delighted Orks will jump at the chance to thump their main rivals and the Eldar, seeing the writing on the wall, will likely take their turn to put the boot in the SMs as well.

After two campaigns of being hammered into the dust, many non-veteran SM players will be screaming about imbalance and unfairness, rerolling and suffering for the first time the loss of morale from being repeatedly on the losing side (something the others will by this stage have got over).

The Orks will then dominate the server until complacency creeps in. Chaos will gleefully try a second Black Crusade, which will for no apparent reason go the way of Abaddon and fall flat on its face. The Eldar will get stomped by someone and no longer be seen as a vital ally and, from the ashes of their failure, the Space Marines will paint their armour black, forswear their Primarch, pull themselves together and rise like a phoenix from the ashes screaming "We are returned" as they barrel through everyone (are you sure about this bit? It sounds strangely familiar, and yet somehow out of place. Ed.)

I'm going to play Space Marines, of course, but I'd be interested in trying out the Eldar faction for a change
If Imperial Guard makes it's way into the Eternal Crusade as a f2p faction, I would definitely check them out even if it means playing "the meat" and cannon-fodder
I don't really care about Chaos and Orks, I might look into them just out of curiosity, but nothing more - they're not my type.

...
I do hope they add Black Templars one day
I'm also looking forward to playing as an apothecary, a techpriest and a pilot
Somebody's got to heal and revive allies, repair those fortifications and turrets and transport our troops

I enjoy supporting and controlling roles

I was under the impression that this was already happening.

In all seriousness though, fairweather faction hoppers are a type of player I hate on a personal level.
The irony is that the most fanatical of SM players will undoubtedly be the first to jump ship to someone else when the going gets rough.

Smooth seas don't make good sailors.

And hilarity ensues!

However, actively trying to circumvent and thwart balance changes, especially if they are needed to ensure fair play, does not speak highly of one's integrity.

I do hope they make you buy the faction pack again and lose subscription to the one you had (without loosing progress) or something like that, so that people stick to the faction they chose (there should be a certain trial period to try out different factions, though)

Or, even better, let people buy as many faction subscriptions they want, but let the transfer happen only once a month

My understanding is this is more along their line of thinking. In the latest AMA session, they answered this question by saying that if you bought the game then no content would be restricted from you, and they understood that people would want to try multiple factions, but they might need to introduce a time barrier to changing between sides.

There are no "faction subscriptions" as such. Buying the game unlocks all four races, or for a F2P player you can unlock a specific race alone.

I'm definitely buying the whole pack just to support the game
Gotta keep a close eye for now, making sure the game meets expectations though
But I'm having a good feeling about all this

Maybe I'll buy a collectors edition..

I wouldn't see a Black Crusade as trying to thwart balance changes. Setting a specific target and organising as much as humanly possible of the race to hit it at the same time is an established (and highly sensible) strategy. If the game allows a population dominant faction to be more powerful than the others, it's difficult to argue its unfair if the lower population factions responds using the same mechanic by being more organised and tactical about deploying in full force to achieve local domination of a certain area against a certain target.

Having a dominant faction get crowd stomped by its competitors because of it bragging about its superiority tends to happen sooner or later. But it's never sustained beyond the short term, because it's an alliance of convenience between the other rivals. Sooner or later one will attempt to take advantage of that (particularly when we're talking about Orks and Chaos) and four-way fighting will resume as normal.

Oh no, you misunderstand. My comment on balance changes was not directed at you, but rather others in the thread who are entertaining such ideas.

Your point on having the dominant faction getting crowd stomped is also facilitated by the presence of the Tyranids, as the dev's are currently playing with the idea that the most dominant/populous faction will also bear the brunt of the Tyranids' focus as a natural difficulty mechanic.