It just is what it is. If I was just out to get merits I'd make tons and tons of posts that most sources would agree with, but I don't because I don't care that much to not just chill and be me.
That is, a good merit source, in my opinion, should merit a post even if he don't agree with, acknowledging the effort from users.
For the rest I agree on the most are you saying here, some posts are over merited for no reason and others are under merited only because you are not on the "good poster" for the source.
We maybe also talk about the reduction of required merit for legendary rank since if someone is lucky to hit legendary around 750-800 activity why he should wait till 1k merit? Maybe having a rank that require merit=activity after hero rank is the way to go.
And for last point, some legendary accounts really don't deserve their rank, they shit spam on the forum while other people with lower rank post better than them and they are still stucked on lowers ranks.