In and of themselves, neither frames or iframes are inaccessible. They tend to cause more issues with usability than accessibility. For example, users cannot link directly to content that a frame or iframe may contain, and that content cannot be bookmarked. Users may also find that frames "break the 'back' button."
When you implement frames, however, you'll want to make sure that you:
Meaningful titles help screen reader users navigate among frames. Titles for orientation are especially important when content in one frame changes based on a change made in another frame.