True, you have to act responsibly to use credit cards effectively.
But in many ways, they ARE the devil even if you act responsibly. Credit card companies have been known to act unethically (to say the least), including doing things like raising the interest rate or adding surprise annual fees EVEN to those who pay off all that they owe on time. And randomly lowering the credit limit. And jacking interest rates from 9% to 32% when someone is 1 day late. And charging a $39 late fee on a balance of $19.
Just saying - they cannot be trusted, which is why Congress has tried, and partly succeeded, in enacting legislation to reign in some of their unscrupulous practices.