It's not that good. It's no bulkier than something like Mew. Using the CB Talonflame calc as a benchmark:
252+ Atk Choice Band Talonflame Brave Bird vs. 252 HP / 252+ Def Latias: 150-177 (41.2 - 48.6%) -- guaranteed 3HKO
252+ Atk Choice Band Talonflame Brave Bird vs. 252 HP / 252+ Def Mew: 169-201 (41.8 - 49.7%) -- guaranteed 3HKO
Mew is more physically bulky when you factor in Leftovers recovery. Latias has higher other stats but when you consider the fact it takes up a mega slot, needs a turn to mega evolve before it can switch in easily, and has extra weaknesses to Dragon, Ice and Fairy, it's really nothing gamebreaking.
I'm actually kind of pleased they seem a bit underwhelming. The metagame doesn't need any more overpowered Megas.