After having done a lot of research on the topic I found the following excellent research piece on ETF.com:
Wealthfront modifies historic asset-class returns with current market
implied expected returns (Black-Litterman) as well as with the
in-house views of Chief Investment Officer Burton Malkiel’s team. In
addition, Wealthfront sets minimum and maximum weights for each asset
type. The resulting portfolio has an unmistakable Malkiel flavor to
it, with an emerging market allocation that reflects his interest in
Betterment uses Black-Litterman currently implied market expected
returns, but deliberately includes small-cap and value as separate
asset classes, adding a classic Fama-French factor tilt. It doesn’t
constrain the portfolio weights, but they do account for downside
risk. Betterment’s portfolios wind up quite similar to the global
market, at least on the equities side.
Covestor deliberately veers away from its optimizer to hedge its
portfolios against inflation and to adjust for downside risk. Its wide
constraints allow heavy weights to emerging markets.
Wise Banyan constrains its portfolio weights “tighter than most,” back
toward market-cap weights, according to Herbert Moore, co-founder and
chief investment officer. This might explain why its portfolios
allocate generously to U.S. equities, and away from the rest of the
global equity market.
Invessence includes the largest number of asset types, adding
granularity to the fixed-income side. It bases asset-class returns
expectations on up to 80 years of historical ETF or index returns, but
uses only nine years of volatility history. Invessence employs gold as
an inflation hedge. It also constrains all asset weights except for
U.S. equity. Sure enough, the U.S. dominates its equity allocation.
FutureAdvisor doesn’t optimize. Instead, its builds its portfolio in
sleeves, creating a glide path much as the target-date mutual funds
do. It builds in a “strategic” allocation to REITs as an inflation
hedge, adding Fama-French type tilts. They’re not kidding. The firm’s
portfolios emphasize small- and midcap stocks, and financials (REITS),
with highest-in-class dividend yields and lowest price/book ratios.
There a many more details here:
The whole 7-parts series on the topic starts here: