Well all I can tell is my ingame Statistic from this, my main Character on the Horde side of the Force.
It counts about 800 Unranked BGs and 520+ Wins from this.
Edit: I made a mistake, it was Arathi Basin, 150Games 100 Wins, not alterac valley.
It is up to objective playstyle, resist to zerg all the time, that you can tutn the tables.
On Alterac Valley i have 156 Games, and 78 Wins to the exact.
So its literally 50%.
Altough it was mostly one sided games.
I remember getting almost every win i got from wotlk to cata, when horde just rushed B and won.
And i remember getting all the losses afterwards in an almost single spree aswell.
Altough I noticed the statistics is incomplete to some degree, wintergrasp is missing entirely for example. But during wotlk, on my old realm, there was no alliance so we won practically every round.
Checking on my Alliance druid, I have 350 BGs, with 238 wins.
The wins spread around all kinds of BGs.
I would say, it is more dependend on what time you play at, and how much you contribute to the game.
This character is low geared, because i started out shadowlands only recently,
Still, from the over 50 BGs i have played now during Shadowlands(i started doing BGs at lvl 58), i have won 38.
I played this Character since almost the release of wotlk in Germany, and always PvPed. I am not a Pro player in any respects, but i kept myself around an 1.9k RBG rating since they were introduced.
So even with this low gear, i still manage to contribute to random BGs, because almost no one else plays the objective.
Due to being Frost, i can still contribute in teamfights with cc, anti cc, and still decent damage, and honestly i even finish high on the boards.
This process obviously gets easier when you gear up even more, and then it starts to really depend on yourself, wether you win your round or not.
I would recommend finding an rated bg group or community.
Cheers Ghazza