Agreed. It may not have many dungeons (I don't know) but the wilderness alone is likely the most hostile.
Argonians have a travel system of roots underground. Chances are that some of the "tunnels" are abandoned or have some sort of a taboo to be considered a dungeon. Think of it like the subway system in Fallout 3.
EDIT: Put my comment to the side, UESP says something a tad different, as well as a taste of what the wilderness is like as the OP was wondering:
By far the most interesting means of transportation in the Black Marsh is by "riding the roots". There is an underground expressway of sorts formed within the roots of the Hist trees, and strange parasitic lifeforms will transport passengers over great distances (while slowly digesting them, but don't worry - that takes months.)[http://www.uesp.net/wiki/Template:Fact] Aside from that, there are caravans, though these tend to be somewhat slow, as the roads in the Black Marsh are typically overgrown, winding, and beset with hostile men and beasts.[http://www.uesp.net/wiki/Template:Fact]
http://www.uesp.net/wiki/Lore:Transportation