There might certainly be disadvantages in the seniority based systems but coming from a company without a seniority system or just looking at friends and family members outside aviation who work for the big 3 consultancy firms I have to say seniority is the better system. No nepotism, no elbows out, no matter the colour of your passport.
I guess it is mostly the pilots who work for the likes of RYR, Norwegian, EK etc that are opposing the seniority system whereas the likes employed by AF, KLM, LH, BA/Virgin and other “decent” employers are pro seniority.