Are Dragon Age fans not guilty of bias? One only need look at Deflagratio's most recent posts to see that isn't true, if anything Dragon Age fans are far quicker to bash other series for things that they would give the Dragon Age series a free pass on.
It's a matter of perspective. People who say the things you agree with are more inclined to say things you consider well founded, logical, appropriate, and reasonable. Those that say things you disagree with are more likely to be wrong, ill tempered, inconsiderate, and hostile.
My experience outside of the BioWare forum is seeing more Witcher fans trash BioWare than the other way around. But I can't say if that's a majority perspective or anything like that. Just that it's my experience. I'll acknowledge that my bias is that I'm more inclined to see those posts, get frustrated, and take note of them (even if I don't respond) as another tick in the box of Witcher fans trashing on BioWare.
That said, if you're coming onto a BioWare message board, I wouldn't be too surprised to meet some resistance. I'd consider it poor taste if I were to head over to CDPR's forums to talk about how much I dislike the game and how they are biased for things they otherwise give a free pass on in The Witcher games.
Which is why I said it reminds me a lot of Nintendo and Sega from when I was a kid. As much time was spent trashing the other as building up your own, when the reality is that they both had games and both provided entertainment and, as far as I was concerned, up to the task of delivering that with a variety of entertainment.
Not everyone is going to like the SNES. Not everyone is going to like the Genesis. But I have a feeling a good number of people would have actually liked what both had to offer if they weren't entrenched in the ideological war. As such, they give free passes to franchise they prefer, and are harder on the one they don't like as much. The issue comes when people evangelize about it, and the dislike for the product is associated with the fanbase that represents it.