Comment 4 for bug 1215002

the user-agent string was recently modified to not use iphone/iphone but instead use
 "Mozilla/5.0 (Ubuntu; %1) WebKit/%2"
where
    // %1: form factor (Mobile, Tablet, Desktop)
    // %2: WebKit version

There are a list of overrides for specific sites found here on the phone:
/usr/lib/arm-linux-gnueabihf/qt5/qml/Ubuntu/Components/Extras/Browser/ua-overrides.js

You can try adding the sites that are having problems to this file, with an Android, iPhone, (or other) override and report back what works best for the sites in question.