The day of the release of the jailbreak tool "evasi0n", it is via an amendment dated on February 3, 2013 of an article published in 2010 on its support website that Apple gives more detail on these risks. It's difficult to know what Apple has changed in the article, but one thing is certain is that Apple gives full detail about the hazardeuse consequences of the jailbreak of iOS devices, including risks of instability, security (vulnerability to attacks), battery drain problem, the poor quality of voice calls and data transmission (eg use of the Internet), the disruption services (such as iCloud) and of course to top it up the unability to install future updates to the system.
Although the U.S. government has reconne by law that the jailbreak was perfectly legal, Apple still consider that it consitutes a pure and simple violation user-end license.
Below is the full article from Apple support site (also available here) :
As designed by Apple, iOS and iPhone, iPad, and iPod touch are designed to work together reliably. Unauthorized modifications to iOS ("jailbreaking") can cause numerous issues to the hacked iPhone, iPad or iPod touch. Examples of issues caused by these unauthorized modifications to iOS have included the following:
Instability: Frequent and unexpected crashes of the device, crashes and freezes of built-in apps and third-party apps, and loss of data.Security vulnerabilities: Security compromises have been introduced by these modifications that could allow hackers to steal personal information, damage the device, attack the wireless network, or introduce malware or viruses.Shortened battery life: The hacked software has caused an accelerated battery drain that shortens the operation of an iPhone, iPad, or iPod touch on a single battery charge.Unreliable voice and data: Dropped calls, slow or unreliable data connections, and delayed or inaccurate location data.Disruption of services: Services such as Visual Voicemail, Weather, and Stocks have been disrupted or no longer work on the device. Additionally, third-party apps that use the Apple Push Notification Service have had difficulty receiving notifications or received notifications that were intended for a different hacked device. Other push-based services such as iCloud and Exchange have experienced problems synchronizing data with their respective servers.Inability to apply future software updates: Some unauthorized modifications have caused damage to iOS that is not repairable. This can result in the hacked iPhone, iPad, or iPod touch becoming permanently inoperable when a future Apple-supplied iOS update is installed.Apple strongly cautions against installing any software that hacks iOS. It is also important to note that unauthorized modification of iOS is a violation of iOS end-user license agreement and because of this, Apple may deny service for an iPhone, iPad, or iPod touch that has installed any unauthorized software.