The second biggest problem with DocBook is the effort needed to convert old-style presentation markup to DocBook markup. Human beings can usually parse the presentation of a document into logical structure automatically, because (for example) they can tell from context when an italic font means `emphasis' and when it means something else such as `this is a foreign phrase'.
Somehow, in converting documents to DocBook, those sorts of distinctions need to be made explicit. Sometimes they're present in the old markup; often they are not, and the missing structural information has to be either deduced by clever heuristics or added by a human.
Here is a summary of the state of conversion tools from various other formats:
The Free Software Foundation has made a policy decision to
support DocBook as an interchange format. Texinfo has enough
structure to make reasonably good automatic conversion possible, and
the 4.x versions of makeinfo feature a
--docbook switch that generates DocBook.
More at the makeinfo project
There is a POD::DocBook module that translates Plain Old Documentation markup to DocBook. It claims to translate every POD tag except the L<> italic tag. The man page also says "Nested =over/=back lists are not supported within DocBook." but notes that the module has been heavily tested.
LaTeX is a (mostly) structural markup macro language built on top of the TeX formatter. There is a project called TeX4ht that (according to the author of PassiveTeX) can generate DocBook from LaTeX.
This is generally considered the biggest and nastiest conversion problem. And indeed, the basic troff(1) markup is at too low a presentation level for automatic conversion tools to do much of any good. However, the gloom in the picture lightens significantly if we consider translation from sources of documents written in macro packages like man(7). These have enough structural features for automatic translation to get some traction.
I wrote a tool to do this myself, because I couldn't find anything else that did a half-decent job of it (and the problem is interesting). It's called doclifter. It will translate to either SGML or XML DocBook from man(7), mdoc(7), ms(7), or me(7) macros. See the documentation for details.