The problem is a wealth tax is almost impossible to implement. People will form crappy charities or put the money in their kids' names or move it to the Caribbean or some other gymnastics that will make worse use of the money overall.
If the wealth tax is uniform it doesn't matter if they put it in crappy charities or their kids names or whatever - the crappy charity or kids will still need to pay the tax.
I see bigger problems with evasion and valuation. Evasion can be solved by coupling the wealth tax with enforcement of property rights. You own an offshore bank account, somebody steals from that offshore bank account, you show up in court to prosecute them, and the government says "I'm sorry, we don't have any record of your ownership of this bank account, and you have never paid taxes on it." Oops. Also makes logical sense, as the function of the state is to enforce property rights.
Valuation is tricky, as a lot of wealth-producing assets are illiquid and it's hard to pin a specific value on them in the absence of a specific transaction. The way LVTs handle this is through statistics: you know what comparable land sells, you know what improvements are on it, you can run a regression against all the features that impact valuation and subtract them out to get a reasonable estimate of the value of the land itself. Something similar could work for income-producing assets: you know all the cash flows from the asset (because you've been declaring your income, right?), you can do a discounted cash flow analysis that smooths them out and arrives at an estimate of the NPV of the asset under current cash flow & interest rate conditions.
Tying property rights to declared wealth only, is how I envision solving this problem as well. The proposal may have flaws, but I suspect it's still better than what we have now.
