日経等のマスメディアがAmazon完全無料スマホアプリの核心を書いてない話

If that sounds kind of shady, that's because it is. Sideloaded apps aren't subject to Google review or automatic checks for malware (Amazon does their own). You can't get automatic updates for security issues, and there's no way to read reviews. It's potentially a hacker's paradise if they can trick you into installing their app.

Normally, then, it's a bad idea to sideload stuff unless you know what you're doing. But, of course, Amazon isn't a hacker. It's a big and trusted company that wants to sell you stuff, and the original app was used by millions.

To be sure you don't open up any security holes, though, make sure that after you make the settings changes needed to get the Amazon Underground app (and any apps you want from inside it), go back in there and change them back when you're done. This will prevent accidentally downloading malicious apps from elsewhere on the web, but you'll have to switch it back when you want to download more from Amazon.