That's easy to answer for Vanguard, which has been around since 1975 and has $3T under management. It's not going anywhere. Both Wealthfront and Betterment were founded in 2008, in Palo Alto and NYC respectively, and have about $2B and $3B under management. I don't think there are any official ratings of probity out there; I'm not sure there's a good source besides trawling through the business press looking for red flags.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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