Ugh, because talking up one perpetually overrated ship wasn't enough for the thread.
The Defiant is a a set of engines with guns strapped to them (or possibly the reverse), and it is impressive that startfleet was able to fit the sort of firepower that's usually mounted on a light cruiser on what is basically a scout ship.
But it's very slow, topping out at like 9.3 or something in an era where most ships can reach 9.6 or faster, and has limited operational endurance, both of which limit the ships effective patrol range. And while it has the punch of a ship several times it's size, it has the shields of a ship about its size. That is a bad combination, it's going to attract a lot more heat because of it's threat level, and it can't survive taking those sorts of hits.
Bear in mind, when the Defiant tried to take on an upgraded Excelsior, a ship that, for all it's other qualities, was still decades older and two generations behind the current standard for startfleet capital ships, the Defiant lost, because the Lakota was able to deliver a kill shot the defiant, but as far as the script says the reverse was not true.
If you want a good startfleet light warship, look at the Saber class, which is faster, just as tough, with better multirole capability and better crew accommodations, all at the cost of some (mostly excessive) firepower.