Somewhat depending on writer, The Guardian is highly biased and fairly inaccurate. I don't think they are filling any type of niche in reporting that we will miss. https://adfontesmedia.com/interactive-media-bias-chart/
Looking at the scatter plot of the Guardian they don't seem to have made any attempt at separating news reporting from opinion pieces. It would be strange for anyone to expect the latter to be unbiased.
The chart has been around for a while and is generally believed to be the leader. It's widely used in academia. It's a good tool but not bulletproof, and they admit the bias of their editors freely.