Also, the Defiant was special because Star Trek is all about plot conveniene and retcons. Watch those DS9 episodes with the big fleet battles and you'll see little Federation ships (escorts) blowing up all over the place.
I haven't had any problems in my escort except when I bit off more than I could chew (tried to take on a +4 Negh Vahr) ... I did fine against +3 ships in several missions.
I havent tried any PvP, though.