There are two ways to "side load" apps: One is to jailbreak your iPhone. Which means any security supplied by Apple is basically gone. If I was working at the NSA and wanted to break into phones, I'd give a little visit to people who developed a jailbreak and make them an offer they cannot refuse. Unless the Chinese got their first. Think about that. Jailbreak is safe as far as you can trust some totally unknown people who may have been leaned on by some government agency, and possibly not from _your_ government.
The other way is to put an enterprise certificate on your phone, which is what you may have if your phone was supplied to you by your company. The company can then put apps on your phone that have not gone through the App Store review process.