I have a 98% (15a/15d/14s) Lugia with ES/SA maxed out that I've used for sweeping gyms when I was really low on spare revives and potions. Here's what I noticed:
1. If you've ever used a Dragonite with dragon tail to clear through gyms and you've been successful at dodging, then you'll do fine with Lugia, as extrasensory has the same move duration as DT. That said, Lugia is like a Snorlax with wings. It's super tanky, so you'll probably only need to dodge charge moves.
2. In my experience, sky attack performs better than future sight for gym battles, simply due to merits of it being a 2-bar move. It also helps that SA got that damage buff, and has an amazingly cool attack animation. For the quick move, I prefer ES over DT due to STAB and better energy gain.
3. Against Snorlax and Slaking, Lugia does fine. Bit of a slow fight, but nothing it can't handle. Chansey is painfully slow. Blissey is a nightmare, and you probably won't want to dodge at all here, because you could risk timing out (note: this assumes you're fighting the normal tanks at high motivation and powered up to high levels. Lugia has a tough time taking on 3k+ Blisseys).
4. Aside from Tyranitar, Lugia also struggles against Aggron due to it resisting both psychic and flying-type moves. Metagross is an annoying matchup as well, but my guess is you won't run into that one anywhere near as often.
Can't think of anything else to note at the moment about Lugia, other than to have fun with it! So long as you're not really pressed on time, Lugia makes for a good tanky option for clearing gyms. Just beware of fully-motivated high CP Blisseys, T-tars, and Aggrons.