It's fine. Pokemon roles:
Ubers-They are the main powerhouse of any team. Since most of the ubers allowed in vgc are speacially based, most are special sweepers: Kyogre, palkia, mewtwo, dialga, lugia, rayquazza sometimes, and the occasional calm mind giratina-(o). Basic movesets include thunder, surf, blizzard, aura sphere, grass knot,ect.
Some can function as mixed sweepers, but it's rare. Rayquaza would be mixed. Rayquazza, as you know, is weak to ice. In this metagame, abomasnows are everywhere and ice shard from weavile can OHKO ray. So with hail and blizzard, ray won't last long unless you can take out abomasnow, and every other poke that learns an ice move.
Physical sweepers:giratina-(o), Ho-oh, Groundon, Rayquazza, and rare bulk up dialga.
Other Options:Lugia sets up screens, but cressialia can also do that while not filling up the 2nd uber position. Though Cressilia doesn't have the stats Lugia does including the 90 in SpA. Giratina and Lugia can be status inducer with T-wave.