Been hearing this a lot on these forums and from ally mercs who are new to pvp.
Below is my data from the last 200+ bgs I have done on IC. I will eventually come back to this post and update and bump if after a month or so (unless I get bored with WotLK).
WSG : Horde Wins -34 / Ally Wins -33
AB : Horde Wins -51 / Ally Wins -28
EOTS : Horde Wins -35 / Ally Wins -22
SOTA : Horde Wins -38 / Ally Wins -19
IOC : Horde Wins -8 / Ally Wins -7
AV : Horde Wins -18 / Ally Wins -13
WG(IC) : Horde Wins -10 / Ally Wins -18
Additional Data :
I play as horde. Abot 1/4th of my bgs I play as healer (disc). All my toons are about 5.5k gs. About 5% of the bgs were ally premades, and about 10% of them were horde premades that my guild and another guild on our server run (usually about 3-8 of us queuing together with little rhyme or reason to comp). Almost all premades were AB, EOTS, WSG, and sometimes SOTA. We win most of our premades. Many of our premades end up matched against ally premades (which honestly are the most fun bgs). Most bgs played around 10pm central time zone. Most games had at least 20-40% ally mercs on horde side except for our premades.
Most, I'd say about 70-80% of random bgs I have been in have been massively one sided, however it's not noticeably one faction more than the other. By one sided I mean really bad, wsgs with ally camping our gy holding our flag or AB that we win by 1350+ etc etc. Only bg I see very much more one sided is SOTA which horde, random or not, win very often, with most of the SOTA losses being ally premades.
Lastly, how can I tell they are premades on ally side? Educated guess when abnormal amount of heavily pvp geared ally from the same server working coordinated with the proper amount of healers present. I say about 5% because some of them could honestly be random groupings of course, but so could some of the horde premades I have been a part of in random bgs.
WG is a wash on IC. Depends on who has the most competent boxers, and ally have more than we do.
So TLDR; No. Horde do not lose all the time or even most of the time (in upper bracket). If you are losing frequently as horde or as a merc, the fault may be at least a little bit on you, sorry. I am not great, not bad at pvp, just an average player in average gear. Yes I've hit up to an 11 bg loss in a row, but I've also seen close to that in W streaks.

