I am wondering how much content you usually need to allow CMS to manage, in particular for user generated content. I think specifically about Sitecore, since I have an upcoming project that will be written with it, but this can be applied to any CMS.
Let's say you had a typical e-commerce site with products for sale, and users could leave reviews for these products, as well as buy products that create orders on the site.
I get the impression by looking at the Sitecore documentation and on the web that you can use Sitecore to manage every piece of content. Each product is its own element in Sitecore, but every time a user submits a review, it creates a review element under this product. When a user makes a purchase, he creates an order item in the content tree.
It seemed pretty awful to me at first. I would just think that you would use CMS for driver content. I could understand that you have products if you need to, but I would put everything else in separte RDb, which is linked to Sitecore through the product identifier. This works more, but right, if everything was hosted on Sitecore, could you have performance issues?
On the other hand, if everything was placed inside your CMS, you would not need to have an additional data layer, and you could probably use the built-in caching of your CMS. It would be much easier to allow your editors to approve or reject reviews, as they could simply manage all this through the Sitecore administrator, rather than create a custom page for him.
Have you worked on sites that have always done this in one direction, or have you tried both? How successfully did you find them?