Great defensive typing in Steel / Flying coupled with good overall mixed bulk makes Celesteela a great tank and a useful check to several OU threats such as Tapu Lele, Latios, Mega Scizor, and Magearna.
It can wear down various threats with Leech Seed + Protect.
Above average offenses means Celesteela can put up a fight and not be complete setup fodder. This is further benefitted by good a STAB attack in Heavy Slam, which thanks to Celesteela's high weight has around 120 Base Power against most targets and useful coverage in Flamethrower and Earthquake that allows it to actively beat opposing bulky Steel-types and Electric-types.
Beast Boost allows Celesteela to further boost its already good Special Defense, or its Special Attack if it opts for a more offensive set, if it achieves a KO.
Its relatively low base 61 Speed leaves it outsped by various offensive threats.
Lack of reliable recovery can be compromising for its bulk.
Vulnerable to being trapped by Magnezone more than Skarmory, as it lacks a form of reliable recovery to make up for not using Leftovers.