Legality

• Despite Apple's claims that jailbreaking violates copyright laws, the U.S. government ruled that jailbreaking an iPhone is legal. Apple still tries to prevent users from jailbreaking their iPhones with firmware updates that restrict the phone's functionality to the original factory settings.
  
  

Warranty

• Jailbreaking an iPhone voids the manufacturer warranty. If the phone is jailbroken and becomes damaged or malfunctions, Apple will not replace it.
  
  

Malware

• The restrictions Apple places on iPhones also function as security to prevent dangerous applications from running. In the past, hackers have exploited security holes in jailbroken iPhones to run malware and acquire personal data.
  
  

Instability

• Unauthorized third-party applications can contain bugs because they are not always strenuously tested. These bugs can cause the phone to run slowly. Bugs can even "brick" the iPhone and prevent it from working at all.