Would there be a drawback of using the same graphical toolkit on every platform?

[emacsq]

Just wondering if a free toolkit like Gtk supports multiple platforms out of 
the box then would it make sense to use the same GTK-based code on every 
graphical platform? So using the same toolkit for everything graphics, instead 
of implementing features for windows, linux, etc. separately?

Is there something which native UI code does in emacs UI which GTK could not 
support?