> Fixed income collective trust funds typically invest primarily in various types of debt instruments, such as treasury bonds, treasury bills, corporate bonds, sovereign government bonds, secured and unsecured loans, and different types of derivatives based on these instruments.

There were some issues with that in The Great Recession: https://en.wikipedia.org/wiki/2008_financial_crisis.

> Disadvantages include less transparency than traditional mutual funds, difficulty tracking performance, and less oversight of management.

So less verifiable.

That's just what you want to see related to money that millions depend on for retirement. /s

https://en.wikipedia.org/wiki/Collective_trust_fund