Low yield or low interest rate is bad for so many things:
1. Banks hoarding a lot of cash that they can't lend out. Even with the extremely low interests.
2. Consumers have a lot of cash in the banks and do not spend. Bad for economy.
3. Businesses do not want to borrow since they are not confident of the near future and demand.
4. Foreign capital (FX reserves) have been staying in US treasuries as the safest reserve currency. But when the economic activities pick up, those reserve capital will flight out of US and this can cause rapid high rate scenario.
5. The extra low interest puts pressure on other financial assets like stocks.
I recall the same thing in 2008 with the previous QE. Fed injected a lot of capital and expected banks to lend out to support the housing market. But banks did not want to take the credit risks. So the capital stayed at the bank and some were used to trade stocks...