The domination of Dellin & Miller can only hide the shakiness of the rest of the group for so long.
The Yankees currently boast one of the best bullpens in baseball. They rank eighth in ERA- at 76 (3.04 ERA) and fourth in FIP- at 80 (3.27 FIP). They strike out a remarkably high percentage of batters and usually do so without issuing too many walks, either. So at first glance, it wouldn't seem like the relief corps should be much of a problem.
However, the group is extremely top-heavy at the moment thanks to its unhittable late-game duo of Dellin Betances and Andrew Miller.