Its half luck, half effort!
50:50? I think it's not like that. Maybe only 10% (I can't give an actual percentage about this) we can get merit based on our luck. Like, we are expected to get 1 merit for our post, but in reality we got 20 merit just from that post. It can be said as luck. Merit that we get is based on effort, even our very good post (on your behalf) not get merit, it's not because we are not lucky. But, merit is subjective as DdmrDdmr says above. Good for us, maybe for another member it just a general post, so we don't get merits. Think about our effort first to get merit. Because merit is not a lottery ticket.
IMO
50:50 is more like a figure of speech. What I meant is that even if you make a really, really, really, useful or constructive or entertaining post it's not certain that somebody will actually bother to merit you.