Sounds like it would be a big incentive for companies to stay private as long as possible.
Many privately-held companies sell stock, and their value can thus be calculated.
For those that do no stock trades, a gross income tax would be a simple alternative.
Seems like a strategy could emerge to do private sales at well below estimated value, in order to game this system. You could make it illegal, but it would be incredibly hard to prove (and harder still to detect).
Seems like such a strategy would already be illegal as a means of avoiding capital gains taxes.
It would also have the side-effect of destroying the company's valuation and thus its credit rating.
While not a viable way to actually do things, it is fun to imagine crazy unicorns like WeWork being taxed on their outrageous valuations.