There’s always the risk that a robo-advisor will go out of business. Do your research and choose a robo-advisor that has a trusted custodian bank. The custodian bank takes over your account if a robo-advisor goes out of business. They can transfer your assets or liquidate them for you.
So while it seems like robo-advisors don’t make money – they have their ways. As with any investment, do your research. Find out what a robo-advisor charges, even if it seems like they don’t charge anything upfront.