Mermaid is lightweight and simple to use to throw together a quick flowchart. The ability to work purely within Docs, rather than importing charts from other software, means it is easy to alter the chart on the go. Don't be put off by the code-like interface- it's intuitive and user friendly, even an idiot like me can make something great with it. The team behind it has done a great job responding to feedback and adding new features too. The charts look fantastic with no effort required of you- they're no-nonsense, stylish and easy on the eyes.
The main drawback to Mermaid is that layout control is very finicky- you can choose the overall direction of the chart or change the order of your syntax which will (maybe) rearrange parts of the flowchart, but that's about it. It can be a bit of a gamble/trial and error to get a layout you're happy with, and the result can be rather sprawling. If you are creating a complex flowchart with a lot of information, you might want to find another tool that gives you greater control. But for most diagrams you'd put in a document, Mermaid is the best solution.