They’re great in theory but you tend to run into issues when there’s lots of data points or dis-proportionally large segments.
Here’s what happens in practice -
https://community.monzo.com/t/monzo-desktop-app-quids/7057/22?u=alexs
& here’s Hugo’s comments on the challenges with them -
https://community.monzo.com/t/where-are-the-doughnuts/5904/2?u=alexs