How confident do you think that robo advisory can do in the long term as compared to the return via S&P500 ETF

Ultimately, the risk is if the price is above what you bought when you want to sell it. Having a balanced portfolio of bonds and equities ensures that in the event you are selling during a downturn, there's still some cushion instead of a straight drop. In addition, other asset classes such as crypto or alternative investments may also outperform equities.