Also important bits:
First, a well-designed wealth tax must apply to all types of assets equally (rather than exempting certain types of assets such as real estate, which would make tax avoidance by shifting between asset classes easy and likely).
Another concern is that the wealthy may move abroad in response to a wealth tax. Even if some do, that does not mean they can avoid the wealth tax. To reduce the incentive, either a substantial exit tax can be imposed or annual wealth tax obligations can continue to be applied after expatriation for a set number of years. This would be a fair recognition of the broader society’s contribution to creating and enabling these fortunes.
A hard cap would also have none of these possible weaknesses, although it may be seen as too radical.