Hi,
Well short answer is that WPM does not support frameworks such as AngularJS and KnockoutJS. Long answer is that only one particular action is not performed correctly in these frameworks and this action is text write action. During text write action several events needs to be triggered (e.g key up, key press, key down, etc). For these frameworks WPM does not trigger some required event to "convince" page, that input element has some text. This is causing behavior that you see during playback that some text is entered to input field, but when you for example click on login button some error saying that you have to fill some field in text box is present. When you use X/Y you are sending key strokes into browser window/page itself, so in X/Y mode all events are triggered correctly by page itself.
Fixing this will definitely be one of the drivers in future releases. I hope it will be the next one, but no promises.
Petr