It is a bit bizarre that in the various discussions of a wealth tax no one mentions the most obvious way that rich people can avoid paying: renounce their U.S. citizenship. This would make them completely exempt from a U.S. wealth tax.
While Warren's proposal, and presumably Sanders' as well, would include a steep exit tax on the wealth of people renouncing their citizenship, this would only apply after the tax is in place. While there is little reason to believe that most billionaires are especially bright, it unlikely that most of them are morons. If they don't want to pay the tax they could leave while it was being debated in Congress, if they thought it likely to pass.
It's not clear how many would choose this route, but many billionaires have made it quite clear that they are not committed to the country and don't have much respect for democracy. If they felt their wealth was seriously threatened, it's hard to believe that many would not simply choose to give up their citizenship.