I'm not clear what the withdrawal process looks like for services like Betterment. Let's say I need some money in 2 years. Do I sell a portion of portfolio for the amount that I need? Can I cherry pick and sell the "winner" funds and keep holding the others?
With robo-advisor services, you don't select which funds you buy or sell. All you do is decide on a risk profile, and when you add or remove funds to your account, they decide what to buy or sell based on that profile.
Each service might decide how to do this differently. Looking at Betterment's case, it looks like they try to do rebalancing when you buy. So if you put a deposit in each month, they'll buy the lower priced funds to rebalance your account to the desired ratio. In a taxable account, Betterment likes to optimize your tax liability when you withdraw, so they sell the funds that have lost money before they sell the ones that have gained.