I've only seen fatebringer drop one time over 10 hard completions and countless non complete runs. Luckily it dropped for me! I also could have missed it in the activity scroll drop thing though so there might have been more. I've seen the vex drop like 6 or so times, but people seem more excited to talk about getting the vex for some reason so that may be why it seems like it drops more. I personally was more excited for the fatebringer but that's just me.