Unauthorized modification of iOS can cause instability, security vulnerabilities, shortened battery life, and other issues

Apple Support:

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.

Unfortunately, Apple Geniuses are prohibited from punching customers who turn up to the Genius Bar and complain why their jailbroken device is getting poor battery life when it’s displaying an animated background.