C2 isn't really meant to build full websites.
As you mentionned, search engines won't be able to crawl your content, embeding content like videos will require you to use a custom plugin like "DIV" to display said content, all in HTML.
Forums, e-commerce...
Considering you have PHP frameworks existing, putting a "C2 frontend" on it doesn't make much sense.
You can use C2 to make some applications/elements of your website.
For example, you could consider making the navigation bar in C2 as long as you know the URLs you're aiming for, that your user's browser is executing javascript and is HTML5-friendly.
C2 isn't really recommanded for entire website.