I would say the problem is slightly more than just weekend work. BA has a very particular ethos and it can be quite depressing as a junior pilot. Basically it seems at times like our only job is to make life better for everyone above you. We get paid less so they get paid more, we get a 34 year pay scale whilst they get 24, our pensions are **** to pay for their massive ones, we work weekends so they don't have to, we do reserve 4 times a year so they don't have to do any, Christmas and school holidays we work so others can spend time with their family etc etc etc. I fully appreciate the "we had to do it argument" but many of us have worked for decades before joining so the idea of being everyone's scivie is a bit hard to swallow.
I knew seniority rules before joining but It been harder for me to accept than I thought, it litterally rules every single aspect of your life at BA and being at the bottom is not great!
All that said it's not going to change and I did kind of know what I was joining, generally it's a decent place to work and I could be worse off.
As for the JSS/Bidline argument I could not care less and I don't suppose many aspiring BA pilots will either.