Those tools are mature 10-20+ year and we're just getting feedback for our little ol' alpha born ~yesterday. The limitations are temporary -- we are working on an offline, locally runnable experience (it's in the roadmap), but, alas, engineering bandwidth of a 6 person team.
But when the project is trying to compare itself to others, it's only fair to compare based on what's true of the project currently vs what's aspirational. I would guess that for many potential users, the temporary limitations trump most of the stuff in the comparison portion of the FAQ, or the "current shortcomings" section.
And the "Getting Started" section could also say "create an account and familiarize yourself with the existing terrastruct product" or whatever.
You can fully modify styles in mermaid using CSS and style attributes. You can also load it as a JS library, attach events to chart components, programmatically render charts plus way more.
edit: I've updated the docs to make this clear