Look at any number of existing games and ask yourself which would be harder to balance, a new unit in an existing faction or a completely new faction? This could apply to any polarized game, like RPGs or RTSs.
When you're talking about incremental changes such as we see in X-Wing - with ship statistics, pilot abilities, upgrades and so on all adding to the overall effectiveness of your force - adding a new unit into an existing faction can be much harder to balance. 2 x 2 = 4, 4 x 4 =16, 16 x 16 = 256... and so on. The more ships you add to a faction, the more options you open up within that faction, and the more possibilities you need to take into account.
I'd argue that adding the Z-95 to the Rebel fleet has changed the game a hell of a lot more than adding the Z-95 to an entirely new faction would have. The Z-95 has changed the way the Rebel forces can be played on a fundamental level, if it was introduced as part of a new faction it's arrival wouldn't have changed the way Rebel lists were played at all.
I think we're in danger of confusing what it means to be unbalanced with a shift in the meta, and to that end your observations about the Z-95 are too narrow. What if this theoretical new faction consisted of all of the wave 4 ships, could you make an accurate assessment of how that would impact the game?
I will have to confess that I'm not as well versed as you are in these other games. Can you give me examples where the imbalance happened for number-of-factions reasons, rather than power-creep-in-pursuit-of-quick-profits reasons?
Is that a subtle allusion to 40K? I'm not too sure that I see those things as being mutually exclusive. FFG is a brilliant company, and they do right by us all the time, but they're far from infallible. I feel like we're visiting all the old familiar places...
God dammit, who the hell started this third faction mess again?
Edited by WonderWAAAGH