They are amazing at flowcharts, because Mermaid is text based
Virtually every diagram it makes that's over 6 nodes becomes a spaghetti mess and you can barely influence layout yourself. The default of using curves, the lack of care to prevent lines from intersecting with labels and even nodes themselves, making a plurality of graph types and shapes before DB ERDs can point arrows from/to a property, docs that aren't versioned...
For something that's become nearly universal and standard, it just feels remarkably half-baked.
The interesting part for me was separating the semantic structure from the layout completely.