i use wacom graphire4 -graphic tablet with mouse : 3 buttons + mouse wheel.
with kubuntu 14.10 (qt4) , it's working flawlessly.

with kubuntu 15.04, wacom mouse wheel is working in non qt5 application, but not in qt5 (so all kde plasma5) applications.
standard usb mouse are working in all application, qt5 or not.

xev show that the events are sent correctly, and that standard mouse and wacom mouse send same events when triggering mouse wheel. Bug should be in the way qt5 is filtering events.