I assume you talking about the "safes" for a single pistol and not a larger box. I have seen both simplex and electronic locks fail on pistol boxes. A company called Fas1 has boxes that include both simplex and key locks so you can use either/both. Some or maybe all vaultek boxes and some hornady boxes also have keys in addition to electronics, so they can be opened if the battery dies or electronics fail. I like the idea of a secondary means of access not so much because of access in an emergency, but because at least you could get the gun out of a failed box without having to resort to extreme methods to do so.
I don't care a great deal about the ability of a small box to resist hardcore attempts to break in, as I assume a criminal would just take the whole box. The role of these things is security from the curious, like kids, roommates, house guests, TSA, etc., and people who might take something if an opportunity presented itself, but are not committed to the theft. That said, the heavy duty simplex boxes from Amsec and Vline seem pretty robust. On the other hand, it is my understanding that given an hour or two, one can run thorough every possible simplex combination and open the box.