Blackreach is not actually an interior, but a worldspace (just like the Skyrim worldspace) with "dark weather". This is to allow the use of Landscape creation.

I've not used it too much in Skyrim, but I have spotted threads mentioning LOD issues, worldspace issues and such.
So if one were to create something about the size of Blackreach, which technical glitches is it likely to encounter, and what should be done to avoid them?

Or is it a better option to just go with a normal interior, using cliff and dirt tiles as ground?

http://www.gamesas.com/topic/1345519-creating-new-worldspaceshow-to-get-around-the-broken-ck-features/

This thread has what has been discovered so far and how to bypass some problems.