... I would love to see some type of analysis of how these Robo advisors are doing compared to a 60/40 AA using low cost index funds.
A few years ago I ran a $100K, two year test drive of Schwab's then-new, free robo. (I like to have skin in the game when experimenting.) Search is not my friend today; I can't find my post about it but I'll report from memory. Overall I felt that is delivered pretty close to passive performance.
As @braumeister mentions, the robo likes to keep cash. I probably filled out their client questionnaire a half-dozen times until I got a proposed portfolio with about 5% cash and 95% equity. To be fair, though, the robo assumption is that it has the investors whole portfolio, so some cash is not unreasonable.
The robo split my $100K among 11 Schwab sector index funds and one VG REIT, the smallest holding. It sure did make investing look complicated. During the period IIRC it made one or two trades and, again IIRC, it let dividends go to cash.
I closed the account after two years mostly because all those tiny positions junked up my reports. My conclusion was that this particular robo would be a good choice for someone who really wanted to be hands-off. The robo world has evolved since then, though, so if I were headed that way today I would try to find a robo without so much cash drag.
(I did have an event that where Schwab earned some extra points: One day I got a call from a Schwab advisor who was asking about the 95% equity concentration considering my age. I explained that this was just a small test and part of a much larger portfolio and that Schwab's concern was appreciated but unnecessary.)
Overall the experience left me quite willing to recommend robos to some people. We talk about the option in my adult-ed investing class.