"It's difficult to accurately value" isn't an argument against taxing net worth. It's like the old (likely apocryphal) Winston Churchill joke, "We already established what type of woman you are, now we are just haggling over price". Just take whatever the proposal is, cut in half, quarter, or whatever fraction you want and you no longer have an argument against it.

Personally my favorite idea for this stuff that I have heard thrown around is to allow people to self value everything. However, that self valuation then becomes a price tag. Let a billionaire's accountants put their own evaluation on their equity in a business. But that becomes a binding offer and some other billionaire could come along and buy them out at that valuation. That creates pricing pressure in both directions, the person is prevented from underpricing their assets due to the threat of another buyer coming in and a person is prevented from overpricing because it increases their taxes. And suddenly all the problems regarding how the government appraises these things disappears.