There’s currently an issue with the referrer policy you set not actually being used on the page, but the main drawback to iframes is that many websites will not support rendering if they’re inside a frame. This is done via the x-frame-options HTTP header, which all browsers support. If a given page returns a DENY or SAMEORIGIN value for x-frame-options, then your web browser will refuse to render the content inside the iframe component in your Perspective page.
There’s nothing we can do to change that - the entire point of x-frame-options is to (help) prevent a class of web security attacks called clickjacking.