is that the S-rank criteria are too hard to understand. They're both opaque and inconsistent, which is a particularly brutal combo. Even missions that have the same general set of challenges will weigh them differently. For example, multiple missions provide bonuses for killing mobs, but some of them weigh speed so high that it's incorrect to pause and kill anything, while others weigh mobs higher so it's worth spending a bunch of time mopping them up.
There's no in-game way to know what a given mission wants from you without running it a bunch of times, and even that's mostly noise—did you get an A rank because you killed too few mobs? Too many? Or did you kill the exact right number but wasted too much time elsewhere in the level?
This isn't such a problem for the early levels, which are relatively short and not too difficult. But once you get into Chapter 2, levels are long and culminate in difficult battles as a matter of course. Blasting through, getting a super clean run on the boss, and then getting an A even so is a real dagger.
All this is to say, I think I'm going to hold off on the rush for S ranks until the wiki has had some time to document the actual criteria. I'm not against a bit of experimentation, but I don't find shooting in the dark particularly fun.