The only time that I had a problem was when I purchased some exibition standard cable,nobody told me that it was only for a single use then they throw the stuff away,anyway I remember the bang ! moisture must have got in somewhere and it blew a big hole into the cable,as I say it was the onlyy time it happened,another interesting story about outside electrics was when I was working for a mobile home company,the showground had outside lighting on each van on show,you pushed the cable through rubber grommets into a metal tube,then slid it up towards the fitting with another grommet a push fit over the top,they kept blowing,investigation showed that condensation was getting into the cable ends,a quick fix was silicone grease,never had a single bit of bother afterwards ?
As mentioned before the golden rule with any outside installation is to have a trip switch,and to test the trip every now and again.
A shed without power is like bread without honey.