To a degree, that's true for 5x anything.

In a Warsong Gulch match, if my other five players can capture the flag at least once.
My team usually does extremely well, because my composition (Pally & 4x DK's) are difficult to deal with, as I play on their half of the middle and prioritize their flag carrier over everything else.
When I have to capture the flag myself, because my team cannot do so, I cannot intercept their carrier too... or if I do, another picks up the flag and goes another route.
And the team is by far the strongest I've played in a PvP environment.
Granted, this is non-rated, so the typical player has far less gearing and typically there is not strong coordination of cooldowns, crowd control and the like.
WSG is around 75% win rate.

Similarly, in EotS...
There isn't really an objective that needs to be done... like in AB where you click a flag and need to stick around.
The team essentially runs from one base they hold to the next.
With the sole objective of killing their players... which is all that is necessary to capture that point.
EotS is close to 95% win rate.

Aside from those two, my 5-box which is in more Conquest gear than Honor gear, I have no better than a 50% win rate (in 10/15 man BG's).
And while I'm not necessarily the best at PvP, by any stretch of the imagination, I do understand the strengths and weaknesses of the team... know my binds, and have a lot of practice with the composition.
And it is a stronger composition.

Any other 5-box group I've run, has been an extreme detriment to my faction in the battleground, in anything other than AV/IoC/WG/TB.
Even AB has sucked... I had one match, where I solo (5-box) attacked their 10 players at Blacksmith, killed them and took it... and killed their entire 15-man team, a minute later when they attempted to take it back from me (no help from my team)... despite having the power to kill any combination of their team, including their entire team (have to love opponents who use brute force/hit point depletion as their sole tactic)... having my five unable to split up, and thus unable to hold more than one of the five bases, resulted in a loss.