Sadly I have experienced several episodes of sudden and complete shutdown of the iPad due to overheating (worse on hot days but can happen anytime)
I have SD on a Tesco Hudl 2, but also run it on a Samsung smartphone in my pocket as a backup. A smartphone is smaller than ideal, but way way better than nothing and fine for a backup.
I do a standard flight plan on a map and use Sky Demon as back-up. It works very well. I would never use any electronic device approved or otherwise as a primary navigation option.
I would agree that reliance on a single method of nav is risky. I constantly cross-check SD against what's outside the window (or with radio nav when IMC). I normally draw the track on a map, and I print out the pilot log from SD (and mark ETA/ATA as I go along). It's daft not to have a backup nav strategy in case SD stops working.