Devil's advocate though: I know someone who works in tax (for a private firm of accountants) and hates it. Too dumbed down, and the bar at which they need to reach out to HMRC for guidance or some other resource is just too low.
I find it often ends up linking through to actual legislature, so perhaps it's just something in the middle that's missing, the 'technical reference' level.
Regarding your devil's advocate example, it's obvious a power user (i.e. tax pro) will have different needs than a layman like me, and it's clear that the sites are built for the latter, which makes way more sense as that's like 99.999% of the users anyways. The power users are probably so rare that just forcing them to phone in makes sense.
[1]: https://github.com/alphagov [2]: https://github.com/hmrc
Happy to be corrected, for the purpose of this thread though it was the design system I'm talking about anyway really though.