I have had 3 cards fail. One cheapy and 2 Sandisc. All were replaced no problem on that score but I did lose work on 2 of the occasions. None of the cards were mishandled and all 3 were less than a week old.
So call me a nervous ninny if you will. Paying clients and past experience tells me to be a little careful.
You bring up another good point. Use your gear for a while before taking it on a paid job. Test, retest, put it through the wringer before using it for important work.
Cards do fail. It's why the card companies have RMA services in place. And I'd like to see some reliable statistics on the percentage of cards that fail. I suspect that number is very low, much lower than cards that get lost, misplaced, accidently written over, etc. And I further when these failures take place. Anyone familiar with electronics know most electronics that fail, fail within the first few days/weeks of use. It's been my experience that once a card works fine for a month or so, you can almost consider it bullet proof.
In contrast.. camera equipment. Their electronic components are in the same class. But their mechanical components fail more are they wear, get dirty, don't get maintained (cleaned and lubed), jolted, etc..
I'll admit, I'm not a "sky is falling" type of guy. I am careful, I do test my gear before taking it on a paid gig, and I don't unnecessarily abuse my equipment. But.. I don't baby it either. My cameras and lenses have scratches, worn areas, and I'll always put 'convenience of use' above protection. I never use the on/off switch either, not for changing cards nor batteries. I don't want to have to worry about it. If a camera ever failed through such use I'd simply look for one that fit my style of use.
With all that said.. I've had very few equipment failures. Much less than you'd expect for someone who routinely drags their gear through the jungles of South East Asia and all that entails.
Failing memory cards? I don't give it a thought. Ever.