For example, AllSides includes Breitbart as a source. While it identifies it with the maximum value to the right, it doesn't give a barometer of credibility. For a reader deciding whether Breitbart is worth reading, one should closely examine credibility, not just political leaning. PolitiFact and Media Bias/Fact Check discussed the credibility of Breitbart at length [0] [1].
Then for ground.news, the way it portrays bias is also relatively simplistic. The most prominent indicator is left/center/right, but biases have a lot more nuance. It's more useful to account for how certain publications take certain policy stances. Leanings can be anti-establishment, pro-establishment, socialist, neoliberal, pro-consumption (e.g. Wired), or anti-consumption, and this information is lost when relying on left-center-right categorizations.
The best way to do this is to manually select a few publications that are high on factual credibility, write down their specific biases or leanings, and compare stories across the personally-curated selection of publications. You then personally have more control and understanding of the articles you are presented.
[0] https://www.politifact.com/factchecks/2020/jul/28/stella-imm...