other than the obvious, the problem i have encountered with this 'once per legacy' thing is that i have now accidentally completed objectives on lesser played alts. the result is a few rarely-played toons with 10000cq each (with no hope of earning more), and toons i play the hell out of with less than 1000cq each (with no hope of earning more). in the end not a single toon will qualify for cq.
i used to grind 100,000cq a week on average, but now... the lesson i learnt is to simply not play alts. diminishing rewards means less play time over all.
the result for non-cq ppl is this is one less tank/healer in the queues.