the emissive feature in Lumen is def problematic in a practical sense. the smaller the emitting object is, the more noisy the gi around it gets. Sure turn up gi quality and the noise goes away but then u lose perf. if you have a flashing emissive material. lumen is too slow to update so it ends up looking swimmy. not to mention that other than obj size and emissive amount/color, there is no control over fall off.
its possible Epic is working to improve some of this 
def gonna try a post material solution
C