Edit: See child comments, I misunderstood. Original post below anyway.

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To me, the diagrams have an uglier default design. A crazy amount of dropdown shadow (in the examples), stretched fonts (front page example). It doesn't matter how many diagram types you support if the generated diagrams aren't good quality.

Mermaids defaults are good enough. They're not amazing, but I'm also not embarrassed to show them to other engineers, stakeholders etc

Edit: KeenWrite looks excellent btw