Unless you duplicate the Windows environment, with most of the issues with it- you have an increasing effort with each step away from something like WINE or Winelib that you go from that.
And it's the bulk of the work- not the tail end of it. It may comprise only 20-30% of the code- but it typically consumes 80% of your time doing it because the conversion from Direct3D to OpenGL is typically straightforward to those familiar with both APIs, but the stuff that you end up with from Visual C++'s encouragement of some very, very bad coding practices and design methodologies ends up consuming most of the time doing. It's easier to write BAD code that mostly works than it is to write solid code that works cleanly all the time. It's not much harder, mind, but many of the developers are seriously pressed for time because of unrealistic deadlines all the time. If it takes 10% longer to write it the right way instead of the half-assed, should work most of the time way- the half-assed way will get done every time. Heh...it'll get fixed on the next patch set anyhow, right?