I'm using FAST with the design system props mapped to Tailwind config for my Blazor app team. This lets them consume the awesomeness without writing JS.
The primary reasons seem to be encapsulating accessibility and styles. There is nothing forcing you to use fast-anchor or divider. The theme is exposed in CSS vars so you can use it from a stylesheet on native elements.
Teams doesn't have a free public mode at this time so it sounds like the best option to me. There are official Microsoft instances of Slack and Gitter as well and teams seem to be given a fair degree of freedom.
The template strings are sort of typed, using tagged template literals. These even come with editor support using extensions previously built for lit-html and the like.
fast-foundation and fast-element are the main point of this. The components are mostly just a showcase. Though the MS-style set of components (https://www.npmjs.com/package/@fluentui/web-components) leveraging FAST will probably end up one of the most consumed products.
The difference is that FAST is NOT a component library. It is a tool for building component libraries. The fast-components package is a showcase of what this library is capable of. You could use FAST to create web component libraries that consume bootstrap styles if you want. Microsoft is using this same base to write web components for Fluent UI (previously Office Fabric): https://www.npmjs.com/package/@fluentui/web-components
Whenever I need anything more capable than markdown I go straight to HTML/CSS. I understand this often isn't an option for technical writers who aren't developers but I'd rather not take the time to learn any niche tools in between. I guess you could say "when you can't markdown, markup!"