It is not safety critical. In the unlikely event of both flight computers failing (many aircraft carry a backup FMC/FMS, but even a single computer is far more reliable than anything made on HN), and the consumer iPad which is barely FAA approved as a backup not working, any pilot can land the aircraft with their eyes closed. You think reference materials for the aircraft are the only thing keeping you alive and everything is hopeless if the pilot loses his weight tables? They're specifically trained when they get type rated and have all of the important figures memorized. Not ideal, sure, but "not safe" is another ballgame.

EFBs have a long history in the cockpit. Charts are required to be carried by FAR, but honestly, a scheduled commercial flight genuinely needing to consult one in anger is very rare. That was the impetus to get the FAA to approve iPads for EFB use in the first place, I think, was pilots making the case that they're needed so infrequently that an iPad is fine.

Threads like this make me wish there was more general awareness of what goes on in the cockpit, because I think it would go a long way toward assuaging unnecessary fear. Aviation is one of those rare regulation success stories where pretty much everything has been thought of.