Is that because the people making the loan aren't sophisticated enough to price those securities correctly in this context and so are being taken advantage of?
> a popular kind of financial engineering among very wealthy people
To use assets without a single universal price as collateral? I think everyone does this.
No, it's because using stock (and other securities) this way allows them to "convert" the stock into money without actually realizing the gains. Thus they derive benefit from the gains without actually paying capital gains tax.