In the old version of the brightness switched smoothly after installing KDE NEON brightness is adjusted in spurts. By reducing the luminance of the passing of time, adding one time becomes maximum.
Can't reproduce in: Plasma: 5.12.2 Apps: 17.12.2 Frameworks: 5.43.0 Qt: 5.10.1 Kernel: 4.14.25-1-MANJARO OS: Netrunner Rolling transitions between brightness levels are smooth
Implemented in Plasma 5.20!