A dark and meaningful tone is hard to hit, but I think it's impossible nowadays to do it in full force. The 90s are over and now too much darkness too quickly just seems pointlessly edgy. I think there needs to be a well crafted spectrum between goofy and dark to make the each extreme effective and not overstay its welcome.
The Witcher certainly isn't devoid of lighter and even hilarious moments. In fact, I'd say that those moments keep the Witcher from becoming too grim-dark. I seem to recall either Geralt or the Bloody Baron (or possibly both) making a comment about how absurdly grotesque the fetus monster is, which is probably a sympathetic nudge from the writers indicating that they too think this whole situation is a bit over the top.
I, for one, thought the Trespasser DLC managed to balance its darker themes with its lighthearted moments quite well. Obviously, mileage will vary from player to player, but I think BioWare can manage to land darker themes if they pace their game correctly and they maintain a strong character focus. I also thought that the Brood Mother was well handled in DA:O. Yes, like many things in that game, it was a bit much, but the silliness of the giant naked horror didn't completely overwrite the eerie feeling I got from listening to Hespith's description of their betrayal and mutation.
Where BIoWare seems to loose their edge is when they ignore their characters and just start telling us things. Corypheus and the Fear demon fell totally flat on me because there was barely any buildup, little to no characterization, and precious few scenes that really show the player the horror of these characters. Effective darkness requires expert pacing and succinct imagery. A deep voice and a creepy face just isn't going to cut it.
While contrast is important and it is important to have lighter moments to offset the darker ones usually the overall tone favors one direction over the other.
But then why do we need the tone to be an extreme? BioWare games are almost always upbeat no matter the scenario, and I think BioWare could do worse than maintain that. They don't need to go "goofy" or "dark," when they can have both in their usual light-but-not-blatantly-comedic tone.