And "automated diversified portfolios" is exactly what mortgage backed securities were a decade ago. Look how that turned out.
Meanwhile, LC was alive in '08, and you can look at how their notes performed. Investors lost single digit percentages. In other words, they beat S&P by a lot.
Comparing performance of 2008 vintage loans with 2008 S&P may not be appropriate.
First the 2008 loans didn't completely pay off until 2011. Second, the loans issued during recession (2008) will perform better during recovery phase (2008 onward). Such borrowers are likely to be with stable credit worthiness due to tighter credit criteria by lenders during recession. Also, as borrowers' economic condition improve with recovery, they are more likely to continue making payments instead of defaulting.
The worst performance for unsecured loans is generally for loans that were issued to borrowers during boom time and as economy is headed toward recession (2005-2008). Any event that impacts borrowers capability to make payment will impact first unsecured loans, credit cards, etc.
Federal Reserve (FRED) has quite a bit of data on different type of debts and how they have performed over the years if you are interested in exploring this segment further. I find the debt/lending/fixed-income/bond market segment very fascinating. It is generally not explored in-depth by most people and very quant/maths heavy.
I had actually typed this up but deleted it because I thought the question was more about a beer-money level enterprise.