The Defiant was certainly a flawed concept. When it was designed the intention was for it to intercept unescorted bombers flying from Germany, the sudden defeat of the Allies in France wasn't anticipated. In this scenario it could have been fairly effective, formating alongside the bomber formations and hitting them from their blindspots. However its very debatable whether a Hurri or Spit couldn't have done a better job even then.
The RAF also misused it, sending it out over the Channel into Messerschmitt infested skies. After a couple of successful sorties, the LW got the Defiant's measure. Apart from the lack of frontal armament, it had a lousy turnrate, so it could neither outurn or outrun its opponents.
The Defiant formations also tended to overclaim drastically, an inevitable result of 2 or 3 gunners all shooting up the same plane. The resultant inflated kill tallies encouraged the RAF high brass keep it in the front line for too long despite crippling losses. In hindsight they should have withdrawn the Defiants to the North of England so they could have freed up a couple of Hurri or Spit squadrons to the South.