Generally, compliance is a big issue for financial advisors. According to Betterment's website, since they started investing real money in 2010(removing back-test) a 100% stock Betterment portfolio returned 8.4% cumulatively versus the S&P 500's return of 13.5% with a lower Sharpe ratio(more variable performance). Underperforming the S&P by 5.1% is pretty significant. At a $10,000 initial investment over a 10 year period would be 35k vs 22k.

Since it's a little tricky to find, if you want to view Betterment's performance to a real benchmark(S&P 500), you have to open their site's performance graph(https://www.betterment.com/portfolio/), click "Additional data, options, disclosures" and then check "View all benchmarks and portfolios at once"