How powerful is a Galaxy class starship?
Not as powerful as a Vengeance class Dreadnought.
Seriously, episodes like "Rascals" and the movie Generations
do not make the Dee look particularly mighty. Then the Odyssey was just used as a giant redshirt to show off how badass the Jem'Hadar were.
Episodes like "Best of Both Worlds" show the ship spamming phasers and photons at once and even turn the deflector array into a super duper phaser cannon - yet they're NEVER used again, even when the ships are fighting for their lives. It makes no sense to see the Dee NOT use spam-fire mode or their Deflector O'Death in situations like Generations