Bill Ackman, the maverick CEO of Pershing Square Capital Management, is one of the world’s most celebrated hedge fund managers. With close to $20 billion in assets under management (AUM) and a cult-like following on social media, Ackman’s plans for an initial public offering (IPO) have attracted quite a bit of attention. However, the expected U.S. debut of Pershing Square USA, a version of which already exists in Europe, is facing some key hurdles. First, Ackman has significantly reduced the IPO size, and now plans to raise $2.5 billion to $4 billion for the fund. This is a far cry from the $2…