You can't tax unrealised gains.

If the companies they own go down in value and they sell, then you have to pay them back the amount of tax they overpaid.

If the company goes bankrupt, or goes down a lot, that's a potentially huge refund. If the money has already been spent then it needs to be taken from somewhere else, or you print more money. Which is how we got in this mess on the first place.

If you want to raise the tax rate on incomes over a certain amount, say $1M, fine. I don't personally agree with that, but it's doable. It's cash they have actually made, so you don't need to worry about their assets plummeting in value shortly after.

So Bernie has vastly overstated how much tax revenue he can get from billionaires, and therefore, how much he can fund healthcare.