How reliable is keeping flight maps on an iPad in the cockpit?
Electronic flight bags are plenty reliable.   I carry a primary and spare iPad mini on long-distance flights, in addition to my iPhone, all of which are loaded with ForeFlight.  I also carry a USB cable and car cigarette lighter adapter with me so I can power the tablets while in flight. I reason that the risk of a failure of both iPads combined was a failure of my iPhone is so low that it’s not even worth worrying about.

Keep in mind paper charts carry their own risks with them.  They’re bulky and take up a lot of space.  You can forget to pack the right ones, flashlights can run out of power making them useless in the cockpit at night, etc.

Airlines are going to have their own SOP’s for what shorts and other navigation aids are available to their flight crews, but by and large most airlines are going over to EFBs, in fact beginning to issue them to their pilots, primarily due to the reduction in weight and other costs associated with traditional printed charts and other resources.