Unauthorized modification of iOS can cause instability, security vulnerabilities, shortened battery life, and other issues. In pretty big letters on the apple site.
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.
Source. [Apple](). [title]()http://support.apple.com/kb/HT3743?viewlocale=en_US&locale=en_US