Christian has noted that WebArchive is the least stable of Apple’s proprietary file formats, as there have been bugs in WebKit. Some of the older WebArchive files captured under earlier versions of OS X may have problems, especially under Snow Leopard.
Dynamic images (especially common among ads) result in online activity.
My usual capture mode is as rich text of selected text and images, as I’m more interested in content than in the page layout.
The one question of mine that (I think) hasn’t been answered is this: if I webarchive the page of a blog, for example, will DT preserve that state? Or will DT continue to “refresh” the webarchive as new posts are blogged, thereby displacing the state (and content) that I originally captured?
Small observation – I checked a handful of webarchives from 12 and 36 months ago. From NY Times and a forum I follow (not the DT forum). Both of them were static - no updated content. In sections of the page where NY Times normally updates the content regularly through the day, the content is still the content that was there when I captured the archives 36 months ago.