I usually don't like playing ping-pong with a bad argument, but Lugia in OU is pretty outrageous. I mean, it's not all about Lugia--your argument/logic is pretty shady even to a noob like me.
CM Reuniclus will always beat CM Lugia because Reuniclus is immune to passive damage while Lugia is not.
That's not how CM wars work. Aeroblast has high crit rate, and let's see if uninvested Reuniclus Psychic/Focus Blast will not be scratching it at all even with more CM's. Aeroblast crit? gG. Not to mention, Lugia can run a Roar-CM set
a la Latias to rub it in your face.
Any set running more than 2 support moves is stopped cold by any any decently fast Taunter or Prankster+Taunt user.
Let's see, the most common taunt users, Gliscor/Skarm/Jellicent, has no chance in hell of outspeeding Lugia and its 110 speed. Thundurus/Tornadus can't really switch into Ice Beams on top of the SR damage.
Any fast Sub user will also use it as a setup bait. Un-STAB'd Dragon Tail coming off 90 base attack will fail to break most Subs, losing its phazing properties.
So you use Whirlwind instead. Problems?
All these sets are easily stopped by any Trick-Scarf user, including and especially Rotom-W who resists any attacking move Lugia commonly carries. Even without a scarf set it can just hit Lugia hard with STAB Electric attacks and can exploit their base HP differences with Plain Split.
You can use Trick Scarf only once. If you miss your target, then GG--especially if that's "your (only) way" of dealing with Lugia. Also, Lugia outspeeds Rotom-W, Roosts away damage while PP stalling Hydro Pump like a baws. Or it can just Toxic and gG. Of course, it's not always going to have Toxic, Roost, and speed & special defense investments all at the same time. Even then, you have teammates to cover your weaks. It's not like you're trying to 6-0 with a wall. That would be painful for both sides. The defensive characteristic of Uber states that if it "walls a significant portion of the metagame." If it walls Ttar, then it's walling a host of other Physical threats.
Unlike ubers, Lugia needs to run max speed EVs + speed boosting nature in OU, which significantly lowers its walling potential.
Unlike Ubers, OU isn't full of outrageous powerhouses, which significantly heightens its walling potential.
And reachzero's post that ginganinja linked to sums it all up, really. It's like saying just smacking a fighting-type on a otherwise blissey-weak team and thinking all your problems are solved--it's not. It's not like those mons have a 100% free switch-in (Reflect/Toxic), and Lugia will have a teammate that covers it.