Some good discussion above; a new angle to add to the discussion:

I am very interested to see what happens to Robos after a significant market correction. Robos don't know their clients beyond a risk tolerance questionnaire. Unsophisticated clients may invest too aggressively and unwittingly participate in the full downside of the inevitable downward market cycles we experience from time to time.

Lawsuits???

Part of the reason investment advisers must charge a fee based on AUM is because liability grows as assets grow.