What are the risks if you install an app that's not from the App Store?
With the improved Masque attack, it is possible to perform many actions, including full kernel and user memoryspace traversal, which would include any sort of temporary or permanent backdoor against jailed iOS, including version 8.2 (i.e., this does not require a jailbroken device). When combined with a shim, this can be a devastating attack against other previously-installed apps, perhaps even without memoryspace access. Futher combined with OS X DLL hijacking (another new attack), things get really clever.

Today, as of March 2015, nearly anything is possible. The entire security of the iOS platform has been undone due to Enterprise certificate attacks (i.e., improved Masque), repackaging apps (e.g., Theos), library shimming (i.e., the load operation), and lazy binding abuse. This could lead to function hooking on a jailed device, which could lead to subversion of many concepts, including subversion of the KeyChain, TouchID, and much more than forensics tools are currently capable of (even assuming a lockdown certificate is set and no recoverable lockdown cert or backup is possible on, say, jailed iPhone 6 with iOS 8.2).