The robo advisors charge an extra 0.3-0.4% on top of Vanguard's expense ratios. So at the very least, from a cost perspective, they're more expensive than just using Vanguard.

(Side note: The target date funds have slightly higher expense ratios than just rolling your own with the same underlying funds and "Admiral" share classes.)

The question is, what is the value you're getting out of Betterment/Wealthfront? I don't recall where, but I've seen one or the other say that its estimated value-add was 0.7-1%. So less their fees, that's net positive (if you trust that).

Automated tax-loss harvesting is a big deal for workers. It saves you a little income tax at your marginal rate.

Automated rebalancing removes some of the annual work associated with maintaining your own portfolio (but so does a target-date fund).

At the end of the day, I'm not sure they're worth the fees... I still manage my own (simple index-fund portfolio). But they're pretty close to worth it.