They have a massive destabilizing effect on the economy, especially if that wealth is tied up into stock of a company
Take for example the "great evil" of today Amazon and Jeff Bezos. If Bezo's was forced to liquidate say 2% of his ownership in Amazon to satisfy his wealth tax that would send the stock into a free fall wiping out many many times more than that in value. So not only does that make it hard to actually calculate the tax bill, over time that kill most investments, 401K's pensions, etc
Taxes on subjective wealth (i.e totaling all assets up based on "current market value") is a TERRIBLE system, and very unethical far more unethical than income based taxation