The problem is that transactions in those retirement accounts can still trigger wash sales that Betterment cannot coordinate. That's why they recommend transferring your IRA and previous employer 401k to them to avoid wash sales.
For your current 401k they don't provide any guarantee, but just advise customers to only purchase target retirement funds in their 401k. If your 401k includes stuff like total stock market, total bond market, or S&P 500, you are almost certainly going to have wash sales, whether you know about it or not.
This is why I haven't made a taxable account with them. Even your spouses account can interfere. They're selling TLH as if anybody can use it but when you get in the fine print it seems like almost no one would be safe turning it on.