Money laundering and tax evasion. For foreign investors, there’s the whole buying-a-visa thing, where you don’t really care whether the project gets built because the important thing is getting yourself and the rest of your money a safe bolthole in the US. For domestic investors, there are the obvious questions about how much of the investment gets kicked back to investor-owned entities, how much investment has to be made versus the amounts reported, and of course what losses get reported versus the amount of money actually at risk.
(I can think offhand – although it would take someone smarter and more dishonest to figure out the details – that you could structure a project where someone puts in money that generates paper gains which get credited as additional investment, and then before paying out the project experiences huge losses. Without the paper gains only a fraction of the losses would be deductible. So for putting in, say, $100K in real money, you get $500K in losses that you can use to shelter other income. The IRS is wise to this kind of thing, but unraveling it takes a lot of resources. Which is why the continual pushes to reduce IRS funding.)