Working for seniority based airline.
Ernest K. Gann wrote that seniority implied the ostrich-like philosophy that one man can fly as well as another but that the system must remain forever, if only to protect the weak (actual wording may be a bit different, as this is re-translated from German).
I'd paraphrase Churchill, seniority may be the worst system, except for other systems that have been tried from time to time. Merit-based systems may look objective on paper but there's always some leeway for those who hand out the merit points. There is no yardstick to objectively measure who's the best pilot and there are too many variables in daily ops to use operational data to determine who's better. I dare say we are barely able to find out who's good enough.
There is a drawback to seniority if your carrier goes bust. That could but is unlikely to actually be fixed by seniority systems spanning multiple carriers. US ALPA studied that in the past but never came to an agreement to actually implement it.