I guess it's because of her overall great stats and high CP. I would mainly compare her to Snorlax. Of course Lugia can't defend which would be her biggest strength but in my opinion Lugia has high dps moves which are way stronger than Snorlaxes moves with an equal attack-stat, so as a tank-attacker she should be way better than Snorlax.
I will probably try it out to power up my Lugia with ES/FS 87%IV since I don't really have a need for one of those birds. I got 4 powered up Flareon, then Lapras and Cloyster powered up plus a Jolteon and a Magneton. Lugia on the other hand adds a new dimension to my lineup even though I have 2 Alakazam (not a huge fan), an Espeon, two Exxeggutor and a Slowbro powered up.
