In my most honest Opinion, they aren't Bias at all, the only people that sees them as bias are the Haters and Fanboys of loosing characters.
Sure they aren't 100% accurate with their research, but let's be honest here guys, is anyone actually 100% on point with these Vs Matches?. The answer is simple, No. Why no? Because characters are developed literally every week (mostly DC, Marvel and Manga characters). So for example Superman vs Goku. Screw attack did 2 Death Battles for these 2 great characters. The first battle had Goku at maximum SS4 because during that time, there was no such thing as Super Saiyan God and etc. So once there was, We were presented with Their rematch which had The God forms of Goku. See where I'm getting with this guys? Now with Darkseid vs Thanos, SA used Canon materials in this battle, key words "Canon Material", so therefore Heart of the Universe Thanos is out of the picture and basically anything non canon is out.
But I'm just going to leave it at that. I don't enjoy typing up too much cause most people would be bored reading such a long explanation.