Been listening to Imagine Dragons for more than 10 years now. Teetering on a two or three-star rating.
Just to get it out of the way, Mercury Act II is an album that is definitely a quantity over the quality ordeal. Most of the "good" pop songs you've likely already heard since they're re-released (i.e: Sharks, Bones, Enemy).
The more thought-provoking tracks are okay but they're more along the lines of singles. There doesn't seem to be a theme besides "I'm sad; pity me" and they're kind of thrown together. This theme is overdone in the industry as a whole but, even disregarding that, once you get through the 26 songs on there, you're burned out and crave something better. For me, that was Night Visions and Origins. Even a little of Hell and Silence EP. I think it's more of the fact that Imagine Dragons have brought so much musical innovation to the table in the past with albums like Smoke + Mirrors, decent albums look mediocre now.
That being said, the tracks in themselves are good. Good harmonies, the chord progression isn't jarring like some of their earlier work, production is good.
I can't bring myself to recommend to others to listen to Mercury in its entirety.