Other than minimum levels of wealth, it just establishes different reporting and structures for the investments (usually things like K-1s, non-traded vehicles, and little to no liquidity).
Madoff is a great example of this - he took advantage of his investors by lying to them about results. He went to prison for securities fraud (I'm summarizing here), and the investors pursued various lawsuits to get their money back.
In the Madoff case, JPMorgan was not only fined over a billion dollars [1], but also sued (and settled) for over 2 billion with defrauded shareholders [2]. This was basically for their knowing about the fraud and not informing shareholders (again, a legalistic simplification).
[1] https://dealbook.nytimes.com/2014/01/07/jpmorgan-settles-wit...
[2] https://www.cnbc.com/2014/01/06/jpmorgan-to-pay-26-billion-t...