The retirement funds are somewhat inflexible if you want to change your allocations, but you could just get a three-fund portfolio instead: https://www.bogleheads.org/wiki/Three-fund_portfolio

The only advantages that I see in Robo advisors are that:

1) They reduce the human impact on the portfolio: If the stock market is crashing you want to regularly rebalance your bonds into stocks to keep your original target allocation. If you need to do this yourself manually there are probably lots of people who won't do this when the market is crashing.

2) You don't need to take care of TLH yourself. But then again TLH is pretty simple to do yourself.

However some disadvantages are:

1) Those Robo advisors are quite expensive. Wealthfront charges an annual rate of 0.25% on top of what the ETF charge themselves. So if you have a $500k portfolio you're paying $1250 per year to Wealthfront.

2) There's a lock-in: Wealthfront's feature to trade individual stocks instead of ETFs sounds nice, but it won't be easy to move these stocks somewhere else and to manage them yourself. And selling all of those stocks won't be an option either, if this incurs capital gains taxes.