As a junior (~3years) LH DEP (80% ish on my fleet), I made sure I fully understood JSS. I put the work in, spoke to the trainers and put a lot of careful time and effort into my bid. I was awarded Bid Group 19...... I am working 'fully' over every weekend apart from one Golden Day. My third BG was feasible but Global Constraints trashed everything. In the last 12 months, the worst I have had is working 3 of 4 weekends and that was due to being 'shafted' by pre-ops (note to self - never tick 'drop below clash'...). On average, and this is not down to swapping, I have 1-2 completely free weekends a month and working another 1/2 a weekend. If Global Constraints keep acting in the way they are, it is untenable for me (I'm a single Dad and can't work that many weekends). I have a plan to leave BA if this continues which I have to enact in May. The clock is ticking for me.
My take-away (yes it is early doors) is think long and hard if you have family commitments and you are joining direct onto LH. You'll stay junior for a lot longer than SH. Also, think when you'll likely get a command and what your commitments will be then too. You'll be junior for a while again...