The speculation online is that it's because they lowered the minimum for their institution class funds, many large employer retirement funds sold their holdings of the non-institution funds, leaving everyone left with large capital gains and hence large tax bills unless you held it in a 401k/IRA.
I find Wealthfront to be overkill, but this is precisely the kind of thing they'd save you from.
One step further is to hold the index funds and bonds yourself, that is not exactly rocket science.
I would also say that if you have 6 million USD that is a bit different than most wealthfront customers I think.
And fwiw, tax loss harvesting sounds complicated, but it really isn't that hard to do. If I notice stocks have gone down a lot recently, I'll hop into Vanguard, and swap anything that is underwater with another similar, but not identical fund. I have one for international and one for US stocks. Took me a few hours to get a system down, and now it's a few minutes to do the harvest once every few years.
This is true only if you invest once in your life and then hold those assets forever. But if you invest every quarter then you can do TLH on those new lots individually. And since those new lots will keep coming, your TLH will always have something to work with.
That is true, but you pay a percentage fee on all of your assets (which is growing) to be able to TLH assets you've recently added (fairly static amount over time).
How much effort do you have to put in to avoiding wash sales?