I wouldn't say that is true. For example even renaming a Window/UserControl in Visual Studio doesn't work correctly. There are a ton of things that could be improved about WPF, not least (off the top of my head):
* Use Direct2D instead of DirectX9. The renderer used by WPF has been shown to be very inefficient compared to Direct2D.
* Multiple selection is a hack.
* Can't bind data grid column properties.
* BasedOn="{StaticResource {x:Type Button}}" -- eurgh
* How fucking difficult it is to save an image?
* How fucking difficult it is to get an image's pixels (how do you work out Stride?)