Specifically: my immediate project is a funny-key-palatte like used in Mathematica. Based on a configuration text file, it shows a grid of buttons and clicking one sticks something on the system clipboard.
The button face text and the clipboard text require Unicode support, since the whole point is to make easy access to things I can't type directly! That is, mathematical symbols, greek letters, funny arrows...
Tk with geometry manager would be ideal to make the grid, and easy to program the application. But it failed the Unicode test—putting a (TM) symbol on a button showed 3 ASCII characters instead, taking the UTF encoding as individual 8-bit characters.