It isn't difficult at all. This is an oft repeated excuse. The rich use their wealth as collateral to purchase things, like homes, companies, investments, etc. and transfer wealth to inheritors with basically no tax. So stop worrying about trying to figure out how to tax unrealized wealth and start taxing the things that unrealized wealth helps secure and purchases. This is one way to force realization of that wealth and thus tax it as well and get it back into circulation.

Right now, the rich are extremely incentivized to remain "poor on paper", so we should remove those incentives.