Have a few mates who are JSS trainers and got another mate who is a BALPA rep. Both say 9 out of 10 are happy with what they got given in January with their first JSS bid. From the 10% who are not happy around 75% made an error in their bid or simply didn't put in the work to fully understand JSS.
I guess it always the 2.5% who weren't happy who scream the loudest. I for one really appreciate seniority, it makes things fair and transparent. Have seen it in previous airlines who rather took Direct Entry Captains than promoting their own staff. In BA at least loyalty is rewarded and everybody begins at the bottom of the long list and works him/herself up. I still remember we got told on day one "In BA you only have to be junior once" this is so true. With that in mind it makes me laugh some choose to be junior for a prolonged period with their career choices and then start complaining about it. I doubt they will get much empathy from anybody they fly with.