To me, Latios is useful for gym sweeping while the other big dragons (Ray/Nite/Mence) should be preferred for raids. Someone on TSR Reddit posted a time to win heatmap of Giratina counters at 5 level increments. A Rayquaza at level 25 has a similar TTW as a Latios at level 40, factoring in no weather or friend boosts.
Source: https://imgur.com/tP0vhVq