Wunderlist also installs the Titanium Framework, which will not be removed if you uninstall Wunderlist with the Mac or Windows Uninstaller. If you want to remove both Wunderlist and Titanium you will have to do it manually as per the instructions below.
Remove this folder on Mac:
Users/Username/Library/Wunderlist/
Remove this folder on Windows:
C:\Users\Username\AppData\Roaming\Titanium\
Please note: Make sure you change 'Username' to your account username on your computer. Depending on your folder settings, these files can be hidden on Windows. To show the listed files and folders, go to folder settings and change the option for showing hidden files. If you only want to uninstall the application data of Wunderlist remove the folder "appdata\com.wunderkinder.wunderlist"