The Tax Foundation has some criticisms.
Wealth taxes are not a new idea—they date back hundreds of years—but have been used less and less frequently. In 1990, 12 member countries of the Organisation for Economic Co-Operation and Development imposed a wealth tax; today, only four do. Countries have dropped the taxes due to the challenges they pose.
First, these taxes are difficult to administer. The uber wealthy tend to have very hard-to-value assets. They own more than publicly-traded stock, such as real estate holdings, trusts, and business ownership interests. It is difficult to value these assets on an ongoing basis. Imagine a large privately-held company—its value could change almost daily. How would the tax handle these fluctuations?
The whole thing is worth reading.