QtCurve can look great under Qt4/KDE4, and has the advantage to provide a more compact layout than is possible with the KF5 versions of the standard Oxygen and Breeze themes.

There is however an unresolved issue with the drawing of round/elliptical and rounded objects, a regression in the Qt5 port compared to the Qt4 version.

See the attached screenshots.

Reproducible: Always

Steps to Reproduce:
Compare the appearance of radiobuttons, checkboxes, round sliders and even pushbuttons in a Qt4/KDE4 and a Qt5/KF5 application both using the QtCurve theme.

Actual Results:  
The Qt5/KF5 application looks strange and sometimes even toyish (rounded "slider")

Expected Results:  
Identical appearance

The drawing code is (almost) identical, and I can find no indication in the Qt documentation that requirements have changed for drawing circular/elliptic things. The result looks like something isn't being anti-aliased, even after turning on antialiasing everywhere where it was being kept off. (IOW, I have not introduced new AA renderhints.) Forcing a pen linewidth different from 1 pixel (assuming that that could improve AA) made no difference either.