Latias and her mega evolution can easily prove to be the highlight of any team if played correctly. Latias has access to strong STAB attacks like psychic, psyshock, draco meteor and a plethora of coverage moves to supplement herself with. Moves like defog, healing wish, and calm mind can change Latias’ role on a team greatly. This also makes her unpredictable, which is always a good thing.
One thing to take Into consideration is the abundance of fairly common weakness Latias possesses. All dragon types should account for the fact that most teams are prepared for them nowadays. Dragon STAB moves are often predictable and make for an easy fairy-type switch in. Most competitive teams run a dragon-type of their own, which can easily counter Latias if they can outspeed her or she's already dropped a few draco meteors. Not to mention that it's virtually impossible to find a serious team without ice coverage, which dragons have never appreciated. To further compound her problems, Latias’ psychic typing gives her even more problems from dark and bug types, meaning common moves like u-turn and knock off will really hurt. Most Latias sets will also lack the tools to deal with bulky steel types, which are incredibly prevalent in the meta.
While I prefer other dragon types personally, Latias certainly has her uses and place in the competitive scene still. In a game with literally thousands of options it becomes about preference in the end ¯\_(ツ)_/¯ Check out these amazing accounts: