If you have a basic site that doesn't work you can open an issue on the repo. If you have some relatively simple site, its useful for the team to know what features that people are using are broken.
Only a few components of Servo that got split out are used in Firefox. A number of them are shared between the 2 (and servo having the smaller team usually follows the Mozilla upstream)
Speaking only for myself, I think a big part of this is that to be comfortable recommending any browser to be used as a typical browser would require a pretty substantial base set of security features/layers. Servo has a few pieces in place but some of those features are only available on some platforms.
It is much easier to recommend it to be used as an embedding solution where the embedder is usually choosing to only render specific pages as an app.
It is pretty exciting, there's lots of big projects constantly being undertaken in the codebase. In terms of being "greenfield" I will say that the layout engine got rewritten starting in 2020, became the default in 2023, and the legacy one got fully removed in early 2025. There's a lot of reorganization and re-architecting going on in the net and storage components as we speak. There's opportunities to redesign systems to align with newer versions of the various RFCs/WHATWG specs. Basically lots of work to do to stay modern.