People buying blank cheque companies without knowing what they're going to get.
There are very good reasons IPOs require all sorts of public documentation and due diligence. All that regulation which SPACs attempt to side-step is there because of previous scams run in just the same style as SPACs.
They are reminiscent of the weird companies formed during the South Sea Bubble e.g. A company for carrying on an undertaking of great advantage but no-one to know what it is
So why would a company ever go the SPAC route? SPACs are all about avoiding the (financial) disclosure required for a company going public the traditional way, and if you take a look at the list of companies getting acquired by SPACs this year, every single one of them has a red flag that would make their IPO risky (see, e.g., We).