Idea for more reliable switch
Not 0 percent, solder joints still break, in fact they're just as likely to break as the soldering connection to the switch itself. Keep in mind there is no such thing as 100% Only disconnecting the negative terminal to the battery isn't a good idea, it makes a short easier to occur because there's one wire that's already connected. If you want to decrease your chance of failure, simply wire two switches, that way you have to use them both. If one fails you'll recognize it when you go to turn it on and can replace it. It's not 100% but the chances of two switches failing at the same time are so remote as to be negligible.