@mystyk Shield Hero resolves everything it sets up, too! (...well, probably; I did drop it so I don't know if it resolves everything.) My issue was not so much that it didn't resolve things, but rather that it didn't do anything interesting with them before resolving them.
I've got no problems with leaving mysteries open for a long time, as long as the journey is interesting. Otherside Picnic is four volumes in and hasn't really unveiled any of its mysteries. One of my all-time favorites is So I'm a Spider, So What? which is mystery-on-fakeout-on-mystery-on-fakeout-on-mystery and is more confusing after 10 volumes than when it started.
I care less about how or how soon the mysteries are resolved and more about how well the story uses them. I care about whether the series develops its characters, settings, or plot beyond the initial presentation.
That's vague, though, so for a more practical explanation: the "Second Read Test." If I'm considering whether to pick up the next volume of a series or not, I'll pick the first volume back up and read it again. If the things I have learned in the later volumes (about the plot, the characters, the setting, etc.) make me see the first volume in a new light, then the series is going somewhere interesting and I keep going. If not, I'm being strung along by a teasebox. Smartphone failed this test for me after five volumes.
I consider this a pretty low bar to clear. It's not that every mystery's answer should be earth shattering as to reframe everything that came behind it, but if a series goes 5 or 12 volumes, there should be at least one development that reframes at least one element in the earlier volumes, enough to make it interesting on a second read.
Bringing things back on topic, Rising of the Shield Hero answers most/all the questions it poses, but those answers didn't mean anything to me. Knowing that Church worships only three of the four heroes and why doesn't change the way I look at the way the heroes are treated in the first volume (they hate him for some reason -> they hate him for crappy reasons). Knowing who and what Glass is and why she's attacking alongside the Waves doesn't change the way I look at the scenes when she first appears.