A jailbroken iPhone inherently involves a greater risk in the event of an attempt to “hack” by a third party. Indeed, distributed applications on jailbroken devices have access, depending on the very principle of the jailbreak. Moreover, this method is not approved by Apple, which considers it “illegal” , the act invalidates the warranty of the device. However, Apple recognizes the process to the extent that after-sale departments mentions the existence of the jailbreak and explains risks.
Software to jailbreak devices is developed quickly, as soon as a new device that supports iOS is available. This is due to the impatience of the vast community around jailbreak of iOS, which often requires the fastest possible availability for the jailbreak. It happens so that, to overcome this expectation, developers often publish unstable versions of their jailbreak software. This software can be developed quickly and its may activation may require some technical experience.
The jailbreak can not break the device. Indeed, it is only a software change that may be reversed by restoring the original iOS on iTunes (there are no irreversible changes). A jailbroken iPhone iOS which has been restored is exactly the same as those which have never been jailbroken (in return of warranty, Apple is unable to know if the device has been jailbroken before). In this case, the guarantee is applicable. For cons, the only risk is that the jailbroken device may fail and the restoration of iOS is no longer possible (eg: can not turn on the camera). Only in this case, Apple may refuse to honor the guarantee.