I wouldn't say offensive Cresselia is underrated, exactly, but rather that it's just outclassed. Offensively, Reuniclus has better coverage, recoil-free life orb, higher spcecial attack, and the doesn't take damage from weather or status effects. Latias is much faster, has better typing both offensively and defensively, and has higher special attack and coverage(Dragon is only resisted by steel) as well.
I want to try Specs Chomp at one point, LOL.
Special Chomp is rare, they wouldn't see a Draco Metoer coming at them :3
For a defensive pokemon, 80 special attack is great. It allows it to do decent amounts of damage to opponents that can barely hurt it.
It can also do decent amounts of damage for offensive pokemon....but unless you're running a mixed set and using Fire Blast to hit Skarmory and Ferrothorn, decent just doesn't cut it. The thing has 130 attack for Arceus's sake! Choice Band would hit much, much harder. Even with Dragon Claw, it'll probably hit harder then Draco Meteor. And physical or not, dragon attacks are still resisted by steel types. To make matters worse, while 102 speed is all it needs, but it's still out sped a lot. This doesn't make Garchomp's physical sets any better, either, but it means you should try and use the Shark to it's pull potential.
TL
R: They won't see Draco Meteor coming, but they will see outrage and send in a steel type anyway. Skarm and Ferrothorn, the two most common, aren't weak to ground anyway.
lol they send in Skarmory and boom that Fire Blast OHKOs
I already mentioned this earlier in this post, but if Skarm is a problem, just go mixed.