At least on Linux, Tk apps still tend to look like remnants from before Windows 95. (Which personally is the main reason I avoid Tk apps whenever possible, including Tkinter apps.)
Do you have to update code to take advantage of native widgets? I use SCID [1] because it's the best FLOSS tool for its purpose, but the interface is still absolutely horrific looking. It doesn't even respect the DPI hints that GTK and QT programs do, so the fonts are too big on my computer.
