While everyone will point to his high MLB ERA, it came in a very limited number of innings. He also showed better form in Triple-A, posting a 3.66 ERA in 17 starts and a 1.74 ERA over his last six outings. A low-risk depth move with upside if he can carry that AAA success to Chicago.
Three premium prospects for a reliever, even one as filthy as Mason Miller, is insane. No team is gutting their future core (including a top catching prospect and a potential frontline lefty) for 60–70 innings a year. This is the kind of package you start with for a young ace, not a bullpen arm.