As you get far enough into a streak, some of your opponents will start to use legendaries against you, and indeed a few trainer names like Demiathena, Placido, and Brady even have the theme of "My team will only ever have legendaries on it". The ones they will sometimes use include: the Articuno cycle, the Raikou cycle, the Regirock cycle (including Regigigas), Latios/Latias, the Uxie cycle, Heatran, Cresselia, the Cobalion cycle, and the Tornadus cycle (in this last case, they're always in Incarnate form). Obviously, it's not going to allow them to use those unless it allows you the same courtesy to match them if you so choose, and it even gives you a bit more freedom. You can run the Therians if you want, and you can also use Tapus, Silvally, and UBs even though none of your tree opponents will ever do the same.
Note that you do still have to respect the species clause (i.e. using one Suicune is perfectly fine, but you can't use two of them on the same team), and in USUM only, the "normal" track (which ends at 20 battles) no longer has any banned species so you can even use blatant "ubers" like Mewtwo, Groudon, and Xerneas for that.