Overall, that sounds like an argument to exclude Mac support, if anything. But I would suggest that if it's so difficult to get the widgets exact, it's better to use a toolkit that can provide widget portability as well as precise implementations that pass through to native OS windowing.

>If you target the native OS widget set, you must have a separate code base for each platform's native widgets.

I want to clarify the subtle distinction here. If your code's concerns are separated, having to directly provide native widgets on some platforms means a different "codebase" for windowing and widgets only, not for everything. You'd still compile your normal code, and use an #ifdef or equivalent to include the appropriate windowing/widget library.