In my experience, state property tax assessments do a decent job at trying to calculate relative values but a terrible job at defining actual property values. Meaning, they may pretty reliably value my house at 10 or 15% less than the house next door based on age or size, but the actual value they put on either house isn't even close to what it would sell for (I've always seen tax assessments come in much lower than market rate).
I don't know how that plays out with mansions though. Whether a mansion is worth $30M or $10M is often hard to predict with the pool of potential buyers being so low.