Failing a medical? Pure and simple, drugs in the urine. The medical is pretty basic - xray for lungs, general blood test (STD/hepatitis/cholesterol, etc), family history, 'can you touch your toes and see that wall and hear this' - flexibility, vision and hearing. If they find anything (barring failing drug or pregnancy tests), you have the chance to correct it. Unbeknownst to me, I once had a UTI and didn't make the urine test . . . just a $4 run of Walmart antibiotics and I was good to go.
For a shoppie - I see pro's being pretty much being OFF in ports (except for a load in morning when the stores are coming in) and but also a con as the sea days are super long - Pretty sure the job is commission based, so if you have a 'cheap' clientele, you're not making the $$$. Shoppies are usually one of the 'fun' groups on any ship.