After reading this I must say you have a good point, however I don't like the idea for one reason, Sleep Talk + Dragon Tail/Whirlwind. Zero Priority Phasing off of a base 110 Speed and epic bulk is just scary to think about.Yes, I do think that it's overrated. Cresselia is kinda in the same boat as lugia. If it were uber, you'd all be saying stuff like "It has 120 / 120 / 130 defenses! It has recovery! It has a spikes immunity! It resists ground and fighting! It can trickscarf, calm mind, psycho shift ect."
Lugia is cress with lesser bulk, flying typing (3 more weaknesses, bug and grass resistance, SR weakness) and slightly higher offenses (90 / 90 vs 70 / 75.) What does lugia have over pokes like cress, bronzong, uxie, xatu, mandibuzz, latias and gliscor? It's typing is awful for a defensive poke, and it struggles to differentiate itself from everything else.
Also, if it chooses to run max speed (base 110 is pretty good I must admit) and HP it's defenses drop from 106 / 130 / 154 to 106 / 85 / 107.
Scarftars Crunch does 212 - 252 (51.1-60.7%) damage to 248/8 Jolly Lugia (248 lets Lugia Switch into Stealth Rock 4 times) and you only have a 33% chance to not get phased out.