The groundbreaking wealth taxes proposed by Elizabeth Warren and Bernie Sanders would face a major obstacle to successful implementation, even beyond the inevitable political and constitutional challenges: the difficulty of accurately assessing billionaires’ assets.

“The fundamental problem with a wealth tax is determining how much wealth a person has,” said Ric Edelman, a top wealth adviser for the ultrarich. “Billionaires have assets in privately held companies, artwork, and collectives that are hard to value.”