[1] How do you handle capital gains tax, when the user withdraws the amount from their account?
[2] How is this different from marcus/ally and thousand others, that provide money market fund accounts and money there is insured by FDIC.
You can configure which of your shares we will sell first. If you set this to Tax-Sensitive, we will sell shares with a low tax burden first, so if you have a greater inflow into your account than outflow, all the shares with a high tax burden will never be touched and they will eventually be classified as long-term capital gains.
2) Money market funds only allow you to invest in low risk/low reward securities. We allow you to invest a much broader set of securities (including bonds, stock, ETFs, etc).