As stated in other threads, it's 4.1666667 UA miles per dollar spent--not 4.77. And that's only in certain categories of spending (grocery stores/gas stations/drugstores). And there's a limit of either 25,000 UA miles or 15,000 CO/BA miles per year per card. You can also opt to get $300 cash back, which is almost certainly worth more than 15,000 CO miles (less clear regarding the 25,000 UA miles or 15,000 BA miles).

And, of course, you need to shop at a supermarket that takes credit cards (not debit cards), which negates the whole point of this thread.