A lot of people think that the Cash for Clunkers program was actually the single most effective element of the recent US stimulation bill. It really did contribute to a burst in actual buying of products, thus, economic activity. Whereas despite all the money made available to financial institutions there did not seem to be a follow-on rise in new or extended credit for businesses and individuals; if anything, it got worse afterward.
What is the argument for why the CfC program was bad or ineffective?
"It really did contribute to a burst in actual buying of products, thus, economic activity."
Which it took from future sales.
Whether it's better or worse than financial institution measures has nothing to do with whether it was good or bad. If they were both bad and it was less bad, it's still bad.
The argument that it was bad is ... the broken window fallacy. Perfectly fine working cars that could have been sold to people who can't afford new ones were destroyed (the engine oil replaced with a chemical solution that destroyed it after running for a short period of time).
What is the argument for why the CfC program was bad or ineffective?