I actually disagree a lot. I like it when casters can feel free to praise or criticize the players' performance – it keeps things fresh. Far too often casts sterilely relay what is happening or only say good things about the teams. That just becomes so boring – might as well mute the stream.

But when its one caster criticizing and the other caster saying "oh come on, it's not that bad" then there's something theres at least some sort of tension for the viewer to engage with. Subconsciously as a viewer you adopt the narrative of the caster you agree with more and can follow things better. In a way it's just like those singing/variety talent shows, where there's always one judge who is the hard ass that is "mean" and another judge who is always nice. This way there's always someone each audience member agrees with.

In this way, the only time overly harsh criticism of the players is a problem is when both casters end up cirlce-jerking about how bad the teams are.