Quick summary from reading up on tons of links and discussions on Betterment, Wealthfront, WiseBanyan, etc.

TLDR: real concerns:

1. You can actually do it better yourself if you put in enough time: both rebalancing and tax loss harvesting (the latter actually only kicks in when there are losses)
2. Betterment just jacked up their fees, and there is no telling whether they will do it again at any point in the future, which leads us to the third point that is more general for all robo-advisors:
3. An argument can be made that the whole robo-advisor business model is unsustainable, especially with low initially advertised fees that are the key competitive edge over an ocean of other funds, choice quotes:

   • Charles Schwab is already undercutting both companies with no fees (0.00%), and despite what Betterment will tell you, it’s a great deal.

   • 25 basis points is not a business model, it’s a temporary growth tactic.

Even shorter TLDR: it’s worth paying them if you really want to automatically rebalance and diversify across a range of funds and ETFs for a still-low fee, and hope they can make their model work out. If you are not particularly bent on having, e.g. a slice of bonds and overseas equities, then VTI/VOO/what have you is the way to go.

(Of course, in the context of present day, this assumes you are either not of the opinion that we are in an equities bubble, or it’s not a concern for your portfolio choice)