The people complaining do. one person, yes one person, if they are terribly geared or just plain terrible, can actually cause their entire team to lose, in fact look at this thread:
http://forums.worldofwarcraft.com/th...26325564&sid=1
the problem is that the people complaining tend to be bad players, or they just stand around screaming at the entire team in chat, or they are horribly geared for a bg. so it just SEEMS like you lose more. you're probly going to pay more attention to how much you lose, rather than how much you win.
TL'DR battlegroups are equal pretty much, just one bad player in bgs can actually ruin the entire game for their team.