- Fees are hidden, nothing is "free" - The time from when you pay until you own the shares is hidden The time from when you "sell" until you have the money again is hidden - They "aim" to replicate the S&P500 but then they can differ. How much and how often. - All risk and volatility is hidden - Security of computer systems. They seem to be brilliant math guys with little computer security experience at all. Bitcoin syndrome.