One major downside of native rendering is the lack of layout consistency if you’re editing natively and then sharing anywhere else where the diagram will be rendered by mermaid.js.

Valid point! Native rendering won't be pixel-perfect with mermaid.js. The trade-off is speed and no JS runtime. For documents staying in Ferrite, it's great. For sharing, we're adding SVG export in v0.3.0 so you can use mermaid.js for final renders if needed.