Joseph Stiglitz believes so, because it results in problems regarding taxation accounting and arbitrage.

In his view, there should be a single tax that accounts for all your wealth, be it assets, capital gains, rent, salaries, large gifts, etc…

The moment you introduce another form of taxation (and still try to keep the previous ‘single’ all-encompassing tax), you have to ask yourself “How will I account for both taxes? How do I avoid adding up twice in theory and in practice?”

This is not a trivial problem, because it often results in people trying to avoid taxes by seeking to use the lower tax.

This is a form arbitrage. Arbitrage is taking advantage of differences in prices in different markets. For example, offshoring is a type of arbitrage.

Arbitrage is seen when imposing import tariffs. People respond by moving operations to different places in the world or, when taxing different categories of products differently, by claiming products belong to different (low-tax) categories.

In effect, your original attempt to fund your public administration through a tax results in people finding ways of avoiding paying that tax.

Hence the recommendation to tax individual people once (per taxation period) for everything that makes them wealthy.

any tax can be abused so there’s no tax that is substantially worse than the others