One challenge is that the larger your portfolio grows, the more you will benefit from leveraging things like asset location and matching different investments to their most tax efficient account type. Access Investing is too generic to account for managing investments across accounts, particularly if you have an employer 401k, RSU/Options account, or a specific investment type account like TreasuryDirect for I/EE-bonds.

That's not an indictment of the robo advisor not being good, but rather it's just too simple to accommodate the complexity that most portfolios naturally develop as you go through life.