There's no downside at all if you don't carry a balance. (Pay your bill in full each month. Otherwise, you still get the bonus points, but the interest fee you pay for keeping a balance is higher than the value of your bonus). I have been a Discover card holder for many, many years. My cash back bonuses throughout the year pay for some nice Christmas presents.
Ps- I am the kind of customer in the minority because the credit card companies Do Not make any money from me.