I started with Wealthfront and Betterment, but honestly I could just buy some mutual funds at Vanguard and pay 4 or 5 basis points in fees instead 25 (and then the same 4 or 5 basis points for what they buy).
ETFs are free to trade on Vanguard, but you can't buy fractional shares and there's slight differences in how values are calculated.
Sure, if you buy VTI, for example. If you want to buy fractional shares, you can buy VTSAX which is a mutual fund. You can buy both of them for free from Vanguard and they both have 0.04% expense ratios.