The mighty Defiant is to Starfleet what the A-10 Warhog is to the US miltary. She may not be as graceful-looking as other craft, but few things can stand up to her attack power. I'll take the Defiant over another starship anyday. It's small size makes it easier to park.
As for how it was handled in First Contact--it did what the writers wanted it to do. I take that portrail as seriously as I take the battle between an old Bird of Prey and the Enterprise-D in Generations. The writers wanted a quick fight, and couldn't care about the actual stats. If Sisko was in charge, and the Borg attached DS9 on that show, the Defiant whould have defeated the cude and done what it was designed to do.
"Announcing your plans is a good way to hear god laugh." Al Swearengen, Deadwood