2. It takes a lot of time and you frequently have to start over once you find out that what you thought works well, doesn't work well for others.
3. From anecdotical experience, I'd say that user interface design isn't what open source developers are interested in. It's a distraction from what they actually want to work on.
Personally, I find Inkscapes UI okay to use. Blender and Gimp, on the other hand, are a horrible, unintuitive mess. Whenever I need to do some image manipulation, I try to get by with Irfan View and Inkscape (even for raster graphics) as much as possible, just to avoid having to mess around with Gimp.