Reinforcement, undercut, and root drop-through during laser hybrid arc welding of steel were studied in dependence of gap width, welding speed, and wire feeding rate. Generalized trends were obtained through design of experiments. Most of the trends could be explained by a mass balance while some parameter impacts relied on more complex mechanisms. In particular, different levels of complexity of parameter dependencies were distinguished, ranging from monotonous behaviour to maxima and to changing signs of the trends. The findings are of high practical relevance to optimize the process with respect to the weld quality. Moreover, the potential and limits of the design of experiments method, of a mass balance, and of the matrix flow chart method are discussed.