Not parent-poster, but I imagine the most difficult cases involve non-public stocks or non-fungible physical assets. Consider the problem of: "Someone purchased an irreplaceable ancient urn for $1m and put their parents ashes in it, what's that in taxable wealth today?"

It's too easy for people to offer hypothetical prices they'll never have to execute on. You could establish a price by forcing people to sell anything to the highest bidder, but that kinda explodes any conventional idea of property, and now the government is spending all its time running a trillion sketchy auctions while no human has time to do productive work anymore because your neighbor is trying to buy your car for $1 and you need to arrange a more-plausible offer before you lose it.