It oughtn't be. You might not like their opinion pages -- yes, they're typically left-wing -- but The Guardian's reporting is absolutely top notch; amongst the best in the world.
No. They constantly blur editorial features and news. The most shared posts are ludicrous hysterical nutters that they give a platform to because it generates clicks.
How do you differentiate between opinionated/politically biased pieces and general reporting ? What's the criteria for evaluating good (ie trustworthy) reporting ? Curious to know.