BLM have the highest burst damage in the game, so of course there will be tradeoffs.
I play DRG have my DL + Relic and have a hard time matching DPS output on trash mobs in AK vs. a BLM with Ifrit weapon and ilvl 50-55 gear. And no, I'm not a horrible DRG, I know my rotations, and I'm pretty tight with them. However, when it comes to boss fights, I will definitely end up doing more DPS as the BLM will burn out his mana pool if he tried to keep up in a sustained fight.

Things BLMs are good at:
Blowing up trash mobs really fast
AoE like a boss
Getting 1-shotted by bosses
TONS OF DAMAGE